I've seen some mod maps, but how do mods work exactly ? Will any files from the original starcraft game, be changed?

SCMLoader just puts the map's priority above the actual MPQs. The effects should not be permanent once you close the map.

I'm not entirely certain whether SCMLoader actually works for version 1.15.2, although I do know that certain changes such as grafting and iscript do not work. Could you post your findings, please?

Heh, I should update the non-FireGraft stuff, since I am able to do so on my own without any help.

ok, so when using a mod, all games (such as melee) are always changed by the mod (except when using that tool)?
So if there is no uninstaller with the mod, the only way to get the original files back is to reinstall starcraft?

No. Mods are MPQ's that sort of "push" away the original files. When you exit the game and then run it using the regular Starcraft .exe everything will be back to normal. No installers are involved whatsoever.

Most mods run from an exe. If you run StarCraft normally, the mod shouldn't be loaded. If any of the mpqs are replaced by mpqs from the mod, then you will need to get the original mpqs and replace them in your StarCraft folder.

Edit: Damn you Hat >:-0

It's usually suprising to people who are used to modding other games, because the case is with most games that you're actually editing the game itself. However, our modding tools are far more high tech, and only edit the memory, as such, as soon as the game is restarted, it "forgets" all the changes that took place. Running a mod will have absolutely no lasting effect on Starcraft whatsoever.

There are two things, however, this should not be confused with, which are adding servers, and downgrading. Adding servers affects the registry (data on the hard-drive) and does effect Starcraft until you undo your editing or reinstall. Downgrading removes Starcraft's up-to-date files, and replaces them with not-up-to-date files, and as such, will be permenant until you upgrade your version, either via the downgrader, or via one of Blizzard's patch.

I'm not sure what adding servers and downgrading is. Are they part of a mod?

Downgrading: Changing your StarCraft version to a lower one ( i.e., instead of going from 1.00 to 1.15, you go from 1.15 to 1.12b, 1.15 to 1.10, 1.12b to 1.11, or whatever you may need to do ) . Since mods will only run in certain versions of StarCraft, sometimes you need to downgrade, in order to play a given one.
Adding servers: you know how you can connect to USWest, USEast, Europe, or Asia Battle.net gateways? Some people have made their own, and by editing certain files that StarCraft reads, you can connect to them. Since the official Battle.net servers require you be running the latest version of StarCraft, but some of these third-party ones don't, they are the only way to play downgrade-requiring mods in multiplayer...