A full backup covers your entire system and all the data you want to protect. It provides you with a very reliable recovery option, but is slower than other backup variants, and requires a lot more in terms of storage space.
An Incremental backup means exactly that - a backup that includes changes made since the last incremental backup.
Then there is what's referred to as a Differential backup. This records only those changes made since the last full backup.
A good backup application will offer all three types.