Terminals
From Halopedia, the Halo wiki
Terminals are apparently an Easter Egg in Halo 3. LIttle is known about them, but to find all of them will unlock the Marathon Man achievement. Little is known about their role in Halo 3, but it seems likely that they fill in the player on Halo backstory, give instructions and furnish advice, much the same way as they did in Marathon. Little is known about where the terminals are likely to be found, but it is safe to assume that there are a large number of them (given that finding individual terminals does not give an achievement) and that most of them are difficult to find (given that the achievement exists at all). No doubt more will be learned about them when Halo 3 is released.
Trivia
It is possible the Terminals might play a similar role as the "COG Tags" in the video game, Gears of War, which could be collected to unlock Gamerpoints
