Empty Cells can be used to create Water Cells (which can be extracted into 10k Coolant Cells), Lava Cells, Bio Cells (which can be extracted into Biofuel Cells), H. Coal Cells (which can be extracted into Coalfuel Cells), Uranium Cells and Near-Depleted Uranium Cells.
Empty cells can be used to store liquids and then be stacked into a pile of 64 for machine use. Most of the time you do not get a empty cell in return when using them in machines. You can fill a cell just like you would with a bucket, or you can craft them with a bucket full of liquid and an empty cell.
Empty Cells, Water Cells, and Lava Cells can be stacked. Empty Cells can be used to directly obtain water/lava sources blocks by right clicking, similar to a bucket, or can be crafted as such:
Note: There does not appear to be a way to automatically fill cells with any liquid other than water. Water Cells can however be mass-produced by setting up a Redpower Deployer in front of an infinite water source, and piping the cells in with Pneumatic Tubes before removing them with Advanced Wooden Pipes.