Fluid

There are currently only two fluids in Minecraft, Lava and Water.

How Fluids Work
Fluids can spread over into another block if two conditions are met.  There is no other type block or fluid there. It is not above the current fluid. 

Certain fluids spread quicker than others.

Fluid Collision
Lava Springs are full blocks of lava. Normal lava is runoff of lava springs.