Are cheats iligel to use on games

Hardly, the moment you buy a game it's yours and how you play it is completely up to you.

Now it IS illegal on most online games if the only cheats that are available are trainers or hacks. If the actual gaming company has built in tricks or hints to the game and you happen to use those that they provide, that is allowed. But not hacking or trainers (IN ONLINE GAMES: Maplestory, Xbox Live, Ps3 Online, Runescape, WOW, etc).

If you mean cheats on flash games, those are legal as well, so long as you don't use any hacks/trainers on them if it's a bought flash game. But if the cheats come built in or the game is free then it is allowed.