Oh my.
Lava is a fluid block. It can be found deep underground in caves and at the very bottom of maps.
Like Water, lava spreads by duplicating itself to open horizontal and downwards squares. Lava is slower, though, and can be easily outrun. If quick enough, a player can block the flow of lava. If the lava wave is large enough, though, a player may not be able to build fast enough. Also like water, lava slows down the player moving through it but it does so to a greater extent. Lava is also more opaque than water and is harder to see through. If water and lava collide, cobblestone is created. This "technique" can be used to manipulate water and lava.
Lava's texture was changed in the 0.0.19a client update on June 20, 2009 and is now animated.
In the survival game mode, lava takes 5 of the player's hitpoints for every second they are exposed to it because it lights you on fire and is extremely hard to put yourself out. It is not possible to drown in lava, as no air bubbles pop up when submerged, but the damage the player takes while swimming inside the lava is often enough to kill the player within a second, even when constantly healing himself. In Indev, if the Hell theme is chosen, most water is replaced with lava.
In Infdev, the amount of damage has been reduced to .5 of a hitpoint being removed for every second the player is in lava, in addition to the damage from fire. Lava is no longer present at the bottom of maps in this mode. Using a bucket to scoop lava will leave a hole missing from the pool as the lava blocks wont completely refill the space you took from. When placing the lava and scooping it up again, sometimes some lava doesn't dissapear.