Entity

Entities are things in the Minecraft world which are not blocks or other objects which fit in the cubical grid. Everything which can move around freely is an entity.

Minecraft also has tile entities, which are blocks which have extra information beyond the 4-bit data field described in the data values article, such as signs and chests. Tile entities are unrelated to the entities discussed on this page.

General behavior
Most entities have these properties:
 * A position, velocity, and rotation (as according to ordinary Newtonian physics).
 * A specific volume they occupy, which is a 3-dimensional box with a fixed height and width (square when viewed from the top, and not rotating).
 * Current health.
 * Whether they are on fire. Fire reduces health gradually and displays flames covering the entity.
 * All hostile mobs will chase after you hoping to do some damage (although Endermen won't attack you until you look at them).

Entities can be pushed around by water currents.

Entities cannot pass through solid blocks. Most types of entities prevent blocks from being placed in the space they occupy, except for resources (dropped items) which are automatically pushed out of the block to open air. Also, if an entity does overlap a block, such as due to falling sand or a door being swung, then it is free to move out of the solid block but not back in. (For example, if a door is closed on you, you can jump up and stand on top of the bottom half-door block, if there is air above the door.)

Entities are lit according to the light level of the block their position is in. For example, if a minecart runs over a non-straight track directly into a solid block, it will turn black (since solid blocks always have a light level of 0), and arrows are sometimes seen to turn black (especially if fired shallowly into the ceiling).

Types of entities
The following types of entity currently exist in Minecraft.

Entities listed as “solid” obstruct the passage of other entities.

The size is in block lengths (usually considered to be 1 meter), and is the size of an axis-aligned bounding box with the specified width in both horizontal dimensions (X and Z) and the specified height in the Y dimension. This box does not rotate as the object rotates visually.

Mobs have ID numbers independent of and overlapping with IDs for miscellaneous entities.

Further notes
Reverse engineering of the Minecraft network protocol suggests that paintings may be entities, but they are unusual in that they are aligned to the block grid and completely immobile. Doors, portals and beds, the other objects larger than a single block, are actually groups of adjacent blocks.

Arrows, TNT, sand, and gravel are assumed to have infinite health because they are not destroyed by multiple TNT explosions (they can be fired out of cannons), nor by being on fire for long periods.

Boats and Minecarts

 * See also: Transportation (contains information about speeds)

Boats and minecarts appear to recover health over time. For example, they can be broken by two arrows but only in quick succession, and minecarts never break as a result of being on fire (though they will break if they remain in contact with lava). (Boats are quickly destroyed by fire.) The amount of wobbling displayed by boats and minecarts when struck appears to indicate their current health.

Sand and gravel
The following description refers to sand for brevity, but gravel behaves in exactly the same way.

Sand normally exists as a block. When a sand block's support is removed, it turns into a sand entity and falls down until it hits another object, at which point it places itself as a block in the nearest on-grid position, or turns into a sand resource if that position is occupied (such as by a torch). While sand ordinarily falls straight down, it can be redirected while falling by explosions.

If there is a boat or minecart in the way of falling sand, it will place itself as a block above the vehicle, but then immediately fall down again, producing a 'hopping' effect. This can be used in the construction of a sand cannon. (Patched in Beta 1.6 and no longer works)

If when the world is created, a cave is generated underneath sand, Minecraft will fail to update the block and the sand will remain floating until a block is removed from under, above, or beside it. This latter fact means that a single falling sand block can trigger the collapse of an entire region of sand; therefore, sand ceilings should not be disturbed unless one is outside the area, standing over a torch, or prepared to dig out quickly once the sand has fallen.

Thanks for that! It's just the anwser I needed.

Riding
Entities may be riding on, or attached to, other entities. When this is the case, only the lower/"outer" entity's volume collides with other things, and only that entity can control movement. The currently possible combinations are:
 * A mob or player in a minecart or boat.
 * A player on a saddled pig.
 * A skeleton on a spider (Spider Jockey).
 * An arrow lodged in a mob.

It is possible to have multiple levels, such as a player on a pig in a minecart. Technically, it is possible to ride any entity (using mods), such as a creeper or an arrow.

Bugs

 * In Creative mode (Minecraft 1.8 and later), the player can interact entities (attack or 'right-click' action) from far away — even if there are blocks in the way. This can be very annoying. For example, when you try to break a stone block, and there is a painting on the other side of the block, the stone will not break, but the painting will. Also, when you try to place a block on a wall, and there is a minecart behind that wall, you will enter the minecart, as if you had clicked it directly, instead of placing block.