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The skulls are used for multiple things. They change campaign mode in some way, often to make it harder or comical. They also increase your meta game score during campaign. You get a achievement for each skull, which adds to your gamerscore. There is a guide on this website, http://www.supercheats.com/xbox360/guides/halo3/skulllocations.shtml, which can tell you all the locations for the skulls. The guide also consists of a list of all the locations of the Terminals. Hope this helps. =)

Skulls just give you gamerscore relly.

Gold skulls unlock achievements when you pick and multiply your time bonus in campaign scoring but silver ones don't.

Hope this helps!

They give you armor in some cases as well.

Well it sort of mAKES THE GAME HARDER BUT IT GIVES YOU ARMOUR OR CHANGES VIEW AND STUFF LIKE THAT.

Actually to get the gamerscore you have to find the skull then hold on to it for a few seconds.a pop -up will apear confirming that you have acheived that achievment.Second skulls make the game harder on you(i suggest you newbies out there not to use them).To activate a skull just push the X button at the screen where you pick which mission you want to play(not while picking the mission).a screen will come up with a bunch od 2-d pictures of skulls.to activate one go down to one of the skulls that are colored in(this means you have found that skull) and push the A button.if two red dots appear in the eyes of the skull you have activated it.Push the B button to get out of the screen.

Gold skulls give you achievements and in some cases , armor.

They give you armor and points.