Fluid

There are currently only two fluids in Minecraft, Magma 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
Magma Springs are full blocks of magma. Normal magma is runoff of magma springs.