The Magma block closely resembles a mix between lava and obsidian, with an edited version of animated lava overlaid by chunks of obsidian. The block will hurt any entities that touch it, like cactus, and there is a chance as they touch it that they will be set on fire.
This can be used for things like mob traps, and is a lot easier to use than lava since it can be moved by pistons, does not flow so does not need to be contained, and can hurt mobs from underneath while they are being carried by a water stream.

Oh, and the block spawns in the nether, much like quartz. It can be mined with an iron pickaxe and above, and cannot be crafted.

Have fun!