Littering is leaving trash such as paper, building scraps, or anything that should be placed in a trashcan on the public highways, recreational parks, beaches, forests, private and public property.
Basically littering is whenever you put something that was yours somewhere other than the appropriate trash recepticle. You can toss something out the window while driving, leave stuff at a campsite, put stuff on other people lawns, leave your waste on the table at a resturant. All of those are littering.