Thermal Vent
Thermal Vents are found in the Molten Front of Firelands, more specifically in the Igneous Depths, Fireplume Peak and the Molten Flow. They are usually found near some ledge.
When you stand in a thermal vent, you'll notice a buff telling you that you can jump much higher and farther than normal, and, well,.. it's exactly what it says on the tin. Basically, when you jump while you have this buff active, you'll be taken atop of the ledge you're currently facing, although some vents are buggy or badly placed and sometimes throw you against the rock - you'll just have to step back a little.
The buff appears to suffer a ~0.5 sec. delay before actually applying when running in.
If you aren't facing a ledge, you'll simply jump very far... possibly landing on a mob you really didn't want to aggro, or flying above a pool of lava.
Real-world equivalent
Thermal vents are places where hot water flows upwards, due to being lighter than surrounding water. In World of Warcraft, they are the same as in real life, but they happen in the air instead of water.