Lava
Lava is molten rock that is often found in volcanic regions or in places where metal is being forged. In-game, lava is a broad term that encompasses the real-life definitions of lava, magma, and molten metal since all three behave identically within World of Warcraft.
Heavily fel-tainted areas can contain fel lava.
Traversing lava
Generally, there is a route over or around lava pools, however traveling through it is often the fastest way. Common shortcuts include Ragnaros' room in Molten Core and lava-swimming in Blackrock Depths from Lord Incendius to The Black Forge which allows players to skip a large part of the instance for a fast run or to complete the Molten Core attunement quest.
Damage
When touching lava, your character will take fire damage every 2 seconds until dead or clear from the lava. It is possible to avoid one damage tic with a well-timed jump into the air, even if it means landing back in the lava. Fire resistance reduces the damage taken.
Swimming in lava
Lava behaves like water in that a character can swim through deep pools or walk in the shallows. Underneath the surface, the breath meter will be displayed and drowning is possible if the character is not killed by the ticking fire damage first. There is almost never any need to actually travel below the surface of the lava, however, so this is a scenario easily avoided.
No abilities or circumstances that would allow a character to walk on water will work with lava. This means that even dead characters will have to swim through it. However, abilities that increase swim speed and allow underwater breathing will work while in lava.
Travel with care
In some areas, lava is used as a border to enclose a region and prevent a player from leaving the map. Players should take caution when swimming through such areas, as the map beyond the lava may be incomplete, possibly leading to areas with no exit to safe land, or even falling through the world.
Fishing
Lava can be fished in, just like water. Most lava contains mostly poor-quality items (a different loot table than poor-quality loot obtained due to low fishing skill), with the occasional fire-based elemental trade goods, or 
[Coal].
Fire ammonites can be fished from lava in Frostfire Ridge.
The 
[Pond Nettle] mount can be fished from fel lava in Krokuun and the Antoran Wastes.
Magma Thresher Pools can be spotted in the Dragon Isles after obtaining a 
[Polished Basalt Bracelet]. They contain 
[Magma Threshers].