Lava is an entity that flows on top on blocks.
When flowing lava touches flowing Water, Cobblestone can be infinitely created and mined. If lava touches the top of water, smooth stone is created. When Stationary Lava touches flowing Water, or vice-versa, Obsidian is created.
When the player touches lava, they will start to take 2 hearts of damage every second. The player will also get the fire animation on the screen, obstructing their view, indicating they are on fire. Mobs touching lava will get the same effect.
Lava also spawns in vast oceans in the Nether, and in Lava pools underground.