Powered mechanisms are blocks which react to redstone power by moving themselves or other entities, or by producing light, sound, or explosions.
Activating a powered mechanism (in this case, a redstone lamp)
Activation: All powered mechanisms are turned ON by:
an adjacent active power source (Exception: a redstone torch will not turn ON a powered mechanism it is attached to)
an adjacent powered opaque block (strongly-powered or weakly-powered)
a powered redstone repeater facing directly into the powered component
powered redstone wire running directly into the powered component (or on top of it, for opaque powered mechanisms)
A powered mechanism is not turned ON by adjacent redstone wire which is not pointing at it.
Redstone lamp
Usage: A redstone lamp is used to provide light.
Effect: While ON, a redstone lamp produces block light level 15.
Considerations: A redstone lamp is an opaque block, so powering it directly can cause adjacent powered mechanisms (including other redstone lamps) to activate as well.
Piston
A piston may be of two types: a regular piston only pushes blocks, while a sticky piston pushes and pulls blocks.
Usage: A piston is used to move blocks or entities. May be part of a circuit's output (such as a circuit built to control a piston door), or may be incorporated within the circuit to change a circuit's configuration (for example, by moving an opaque block over a redstone torch or away from it).
Placement: A piston has a stone "pushing" component and a wooden "arm", and can be placed so the arm faces in any direction (its "front").
Activating a piston by piston-connectivity (note that the piston on the left is not powered by piston-connectivity because the redstone wire is running past the block above the piston, rather than directly into it, and thus would not power a mechanism there)
Activation: In addition to the methods above, a piston can also be turned ON if one of the methods above would activate a powered mechanism in the block above the piston, even if there is no powered mechanism there (even if the block above the piston is air or a transparent block), but only when the piston receives a block update. This is known as piston connectivity (the piston is "connected" to the block above for the purposes of activation).
Effect: When turned ON, a piston pushes the block in front of its arm, and up to 11 more in front of that (up to 12 blocks total). When turned off, a regular piston pulls its arm back (leaving an air block in front of the piston), while a sticky piston pulls back both its arm and one block (leaving an air block on the other side of the pulled block).
A moving piston or block can also push an entity such as a mob or item.
Some blocks (bedrock, obsidian, chests, etc.) cannot be moved by a piston. Other blocks (flowers, leaves, torches, etc.) will be destroyed but may drop items (as if destroyed by the player). For full details of how pistons interact with other blocks, see Pushing Blocks.
Considerations: When a sticky piston is activated by a 1-tick pulse, it will push a block in front of it, but will fail to pull back the pushed block on the same pulse. If a sticky piston is activated by a 1-tick pulse when there is no block in front of it, it can pull back a block. Thus, a sticky piston running on 1-tick pulses will push and pull a block every other pulse.
Dispenser
Usage: A dispenser is used to provide items, or to create or destroy water and lava.
Effect: When turned ON, or if any redstone update occurs within two blocks while it is ON, a dispenser will trigger. The effects of being triggered vary with the items in the dispenser -- see Dispensing.
Considerations: A dispenser is an opaque block, so powering it directly can cause adjacent powered mechanisms (including other dispensers) to activate as well.
Usage: A rail is used to switch the track of a minecart.
Placement: A rail can be attached to the top of any opaque block, or to the top of an upside-down stair or upside-down slab.
When placed, rail will configure itself to line up with adjacent rails, powered rails, and detector rails, as well as such adjacent rails one block up. If there are two such adjacent rails on non-opposite sides, the rail will curve from one to the other. If there are three or four such adjacent rails, the rail will curve between two of them (when choosing which directions to curve between, a rail "prefers" south over north, and east over west). If there are no such adjacent rails, the rail will line up in the north-south direction. If a rail it would line up with is one block up, a rail will slant upwards towards it without curving (with multiple options to slant upwards to, a rail "prefers", in order: west, east, south, and north). Other configurations can be created by placing and removing various rail.
Effect: While ON, a rail in a "T" junction flips to curve the other way (powering a rail in another configuration has no effect).
Powered rail
Usage: A powered rail is used to propel a minecart.
Placement: A powered rail can be attached to the top of any opaque block, or to the top of an upside-down stair or upside-down slab.
When placed, a powered rail will configure itself to line up with adjacent rails, powered rails, and detector rails, as well as such adjacent rails one block up. If there are two such adjacent rails on non-opposite sides, or three or more such adjacent rails, a powered rail will line up in the east-west direction. If there are no such adjacent rails, a powered rail will line up in the north-south direction. If a rail it would line up with is one block up, a powered rail will slant upwards towards it (with multiple options to slant upwards to, a powered rail "prefers", in order: west, east, south, and north). Other configurations can be created by placing and removing various rail.
Activation: In addition to the methods above, a powered rail can also be turned ON by other adjacent active powered rail, but the power decreases with distance from a power source. Powered rail can transmit power up to 9 rails (the first originally-powered powered rail, and up to eight additional powered rails). Power transmitted in this way cannot power any circuit components except powered rail.
Effect: While ON, a powered rail boosts the speed of a minecart passing over it, or starts a minecart moving away from an adjacent solid block it is in contact with.
Door
A door may be of two types: a wooden door can be opened and closed by redstone power or by a player right-clicking on it, while an iron door can only be opened and closed by redstone power.
Usage: A door is used to control or prevent the movement of mobs, items, boats, and other entities.
Placement: A door can be attached to the top of most opaque blocks, or to the top of an upside-down slab or upside-down stair.
A door is placed on the edge of the block facing the player. By default the door's hinge will be on the left side, but another door or block can force the hinge to the right side.
Effect: While ON, a door re-positions to the other side of its hinge, allowing movement through its former position and denying movement through its current position. When it turns ON, any entities on the door will fall.
A door doesn't actually "move" (the way a piston arm or a pushed block moves), it simply disappears from one side and re-appears on another, so it won't push entities as it "opens".
Trapdoor
Usage: A trapdoor is used to control or prevent the movement of mobs, items, boats, and other entities.
Placement: A trapdoor can be attached to the top or the bottom of the side of most opaque blocks, as well as to glowstone, slabs, and stairs.
Slabs lack a full-side interaction mask -- to attach a top trapdoor to a bottom slab, or a bottom trapdoor to a top slab, attach the trapdoor to another block and then use a piston to move the trapdoor into position with the slab, or move the slab into position with the trapdoor.
Effect: While ON, a trapdoor re-positions itself in a vertical state, allowing vertical movement through it. When it turns ON, any entities on the trapdoor will fall.
A trapdoor doesn't actually "move" (the way a piston arm or a pushed block moves), it simply disappears from one state and re-appears in another, so it won't push entities as it "opens".
Fence gate
Usage: A fence gate is used to control or prevent the movement of mobs, items, boats, and other entities.
Placement: A fence gate can be placed on the top of most blocks. Once placed, the block beneath it may be removed without popping the fence gate.
Effect: While ON, a fence gate re-positions its two gates to either side, allowing movement through it. When it turns ON, any entities on the fence gate will fall.
A fence gate doesn't actually "move" (the way a piston arm or a pushed block moves), it simply disappears from one state and re-appears in another, so it won't push entities as it "opens".
Unlike a door or trapdoor, while ON, a fence gate is completely non-solid (lacks a collision mask) to all entities.
Note block
Usage: A note block is used to produce a player-chosen sound.
Placement: After being placed, a note block's pitch can be adjusted over a two-octave range by right-clicking the note block, and it's "instrument" can be adjusted by placing different blocks beneath it.
Effect: When turned ON, a note block produces a sound. A note block must have air above it to activate.
Considerations: A note block is an opaque block, so powering it directly can cause adjacent powered mechanisms (including other note blocks) to activate as well.
Activation: In addition to the methods above, TNT can also be activated by fire and explosions.
Effect: When turned ON, TNT ignites and becomes primed TNT, an entity which can fall like sand or be pushed by pistons (but isn't moved by water). Primed TNT explodes 40 ticks (4 seconds) after being ignited by redstone power (10-30 ticks for TNT ignited by an explosion).
Command Block
Usage: A command block is used to execute a server command.
Placement: After being placed, the player can set the command to be executed by right-clicking on the command block.
Effect: When turned ON, a command block executes a specific player-defined command once.