both do the same task..! in the sense :wq,:x and ZZ

are used to save the changes of edited to storage and quit..

but to jot down the difference a foolish one.. :wq and :x

requires <enter> key to be pressed to fire this commands

where as ZZ works as a shortcut (hot) key.. it some what

like for prior commands we are requesting Vi to do the work

and later command we are telling/commanding