You find memory addresses that hold a certain desirable value, then convert the address to a GameShark code address that points to the same thing, while also defining whether you will be modifying two or four bytes for that address. The other four digits correspond to what you want the value of those bytes to be. Using a combination of lines to modify multiple addresses, you can achieve many desirable effects, such as loading different actors, stacking unlimited ammo effects, and many, many more. In fact, GameShark codes are probably second only to hex editing in terms of flexibility of all the things you can do (and call me an idiot all you like, but I'm grouping ASM/C hacking in with hex editing on that note).
Also, to back up IvanX, this forum is extremely inactive in the hacking section, so don't expect prompt replies (if any replies come up at all). They also hate multi-posting, so avoid that too if you want to stick around.
I have a few MIPS assembly notes (something like a 9 MB PDF document, so I guess more than a few, heh) if that's what you're talking about. I can upload those, if you like, Punk, and post a download link.
Also, no problem. I just try to help whoever I can.