Lava is some pretty fiery stuff. Fall into it, and take massive damage. Even if you have fire resistance (conferred by a favor by NR, a deity), walking through lava is slow, and if you're not careful you'll still drown. Each turn you are in lava, you take 1000 damage (this is unreducable unless you have fire resistance), and are set on fire for 3 turns. The flat damage additionally doubles every consecutive turn you are in lava.
Yeah. Pretty dangerous.
Nevertheless, lava has its uses. You can make Obsidian by pouring water on it, and you can scoop lava up with some form of bucket. The bucket should be pretty durable, though, or bad things will happen.