Hazards

Hazards are common dangers or things that can cause injury/death.

Danger levels are comparative - the lowest is one.

Physical Hazards
Mobs

Found at all heights from the base of the world, although most common at water level.

Danger varies depending on mob type, see Zombie, Skeleton, Spider and Creeper.

Best way to counter the danger is to use walls to block their movement (and attacks in the case of skeletons), use armor to absorb some/most damage, be well-armed, and survey surroundings carefully.

Danger level: 3 (although greatly varies on time and location)

Note: Forest areas are both good and bad in respects that 'charging' enemies (all but skeleton usually) may get stuck between trees/leaves, but the player may also get pinned in areas while attacked.

Water

Found at all altitudes but in greatest quantities around the main landmass.

Danger is from drowning - any time there are two water blocks in depth there is the risk of drowning.

Best way to counter the danger is to move around the water rather than through it, to keep jumping which makes the player swim on the surface or to build a bridge through the water. Ponds may be drained with buckets.

Danger level: 1

Lava

Found usually below water level, further below water level exponentially increases the chance of finding lava pockets.

Danger is from contact - players are set on fire and require water to extinguish themselves. Lava pockets are often difficult to detect before being tunneled into, although as long as you do not mine directly up or down you will usually be able to outrun the lava flow.

Danger level: 3 (due to the rapid and often unstoppable damage if contacted)

Note: Using water the lava can be transformed into harmless cobblestone or obsidian - players should be wary of digging through or around either of these in case the lava is more than one cell deep.

Gravel/Sand

Found at all altitudes - sand anywhere from about 5 cells below water to varying heights above, gravel can be found at all altitudes.

Danger is from gravity - these blocks are affected unlike most, and undermining them can cause suffocation if trapped.

Danger level: 1 (players can usually just move to one side or another to escape)

Falling

Falling damage can be caused by jumping of a cliff (whether it be deliberately or accidentally) it can cause many damage.Fall damage can be calculated using this formula: FD = (number of blocks - 3) / 2 It can be avoided if Health and Damage
 * Sneaking (default LSHIFT) will prevent you from falling off most blocks.
 * Having some amount of Armor may save your life from otherwise fatal falls.
 * Landing in Water will reduce fall damage. Depending on the height of the fall, it needs up to 3 blocks of water to reliably nullify the fall damage; however, even 1 or 2 blocks may sometimes do so.
 * Moving into a Ladder's area of effect during freefall will reduce vertical velocity to normal ladder descent speed and no damage will be taken. Care must be taken to avoid landing on the narrow top surface of the ladder.
 * Moving into water during freefall will also reduce velocity.
 * Move passive Mobs away to avoid an unpleasant push to your death.
 * Riding in a Minecart, Boat or on a Pig will allow you to leave the scene unhurt.
 * You might not sustain any fall damage while you're on Fire.
 * Jumping counts as falling for 1½ blocks, so keep this in mind when trying to avoid fall damage.
 * Do not dig directly downwards without knowing what is underneath. Empty caverns are randomly placed around the world, and it's quite possible to fall into them (perhaps from a great height, or even onto a Dungeon or Lava!).
 * Don't attempt to jump gaps across long drops. Instead, sneak to the edge of your current platform, and you will be able to place a block beside it; in this way, bridges can be constructed safely.
 * With good timing, you can place a block on the ground under you while falling. This will only reduce damage by a little, but will come in handy if you need to fall 4 blocks, as it will prevent all damage. If a vertical surface is available, a block can be placed on that as well with practice.

TNT

Only man-made, although if placed/detonated incorrectly is incredibly dangerous.

Danger comes from both the explosion and the resulting crater, which can deposit players in deep caves, causing fall damage.

Danger level: 1 (if placed correctly); 4 (if placed wrongly), 1 (fall danger)

Other Hazards
Dungeons can be particularly dangerous - bursting into one by accident with TNT can spell not only a single death but also a mob-ridden section of your base. Most people consider dungeons worth raiding however, because if lucky you can get stocks of redstone dust and gunpowder, saddles which are uncraftable, and music discs, which are otherwise very difficult to obtain.