Fluids are liquids or gases. The only liquid available in Factorio <0.9 was Water, this docu is about that.
In 0.9 new liquids like crude oil or petroleum gas where added. Because some of those mechanics may change in the comming bugfix-versions, this documentation is only very provisional.
Types of Liquids
Using Water to Generate Power
You can use every type of liquid, but hot water is the obvious solution.