Not much, except they provide you really cool information, and if you find them all you will get an achievement.
there are no terminals in halo reach did you mean halo 3 or anniversary
there isn't any
The Seventh Column. Bungie has a thing for sevens.
Random journal entrys from the forerunners
3 each on the ark and on the covenant...1 in halo...to get credit, press a as many times as you can, wait for it to lock up, and keep waiting...then when 2nd message comes up press a to end of that, then exit with b.
when you activate them all you get the achievement called Marathon Man. It should unlock the armor called Security Shoulders
download the map pack
You go to it and hold the X button
There are seven. For help on how to find them, go to the following link: http://www.gamefaqs.com/console/xbox360/code/926632.html
The terminals are for an achievement, Marathon Man (40G), and also for backstory and information of the game. They look like a silo and when you get near it you see some sort of giant eye. The Arbiter cannot view it if you are playing co-op.
If you collect them all in order in one campaign play through (as in on one save, doesn't have to be all at once) you will get an achievement called marathon man
get all terminals collected ( the big shiny silver things which open when near). if you want help my gamertags cosmeticbobbob.