Fuel Cans are containers made of Tin, which are used for storing fuels. They were added by the IndustrialCraft mod. Once filled with fuel, they can be placed in a generator to produce EU, and the empty fuel can will remain in the generator when the fuel is spent -- you can pull the empty can out of a Generator by attaching a pipe to the backside of the machine.
Fuel cans are filled using the Canning Machine to add 6 fuel cells to an empty fuel can. The classic IC2 fuels are biofuel (26,000 EU per fuel can) and coalfuel (76,440 EU per fuel can).
If your modpack also has GregTech installed, a wide range of new fuels can be canned using the Industrial Centrifuge. The amount of EU produced will vary depending on the recipe. The amount of EU will be shown if the player mouses over it in his/her inventory (if you have the "show burn values" option selected in your NEI settings).