Leaf traps are located in Tirannwn.
These are identified by looking just like trees from above. You will see some leaves on the ground. If you step on them, you'll fall into a pit and lose 150 lifepoints. Click "jump over" on the same side to you of the trap. You may fail. If you do fall into one, click on one of the walls closest to you to climb out.
There currently is a glitch when you fail to jump over the trap you will become stuck in a black space, much like the unknown all around Runescape. This is usually fixed by logging out and logging back in.
There is also a potential (it is unknown if this is a glitch) for a poisoned player to be cured by failing the leaf trap.