If you need a definition of a nut you probably shouldnt be attempting to work on a motorcycle. There are many types and sizes of nuts and you can find guides to the various types and sizes on most of the suppliers websites - its often best to read these as it will help to work out their ordering systems. Some parts lists will specify the type of nut and its size (e.g. self-locking M8) - the morini parts list goes further with some acronyms which I havent been able to decipher yet (eg VT. 322). The type of nut required will depend on the application, ideally you shouldnt reuse self-locking nuts.