In this awesome five-part series, the Achievement Hunters break down how to find every terminal and skull in Halo: Combat Evolved Anniversary. Skulls have been gameplay modifiers, and have been a part of the series since Halo 2. Basically, you start a single player level off with a bunch of these guys on to drastically increase difficulty, change the way you play, or induce silliness. The terminals are a fairly recent addition to the series, and give a bunch of back-story on the different characters and conflicts within the universe.
If you find every skull and terminal in the game, you'll get a total of 170G and six achievements. Those achievements are 'What Have We Here?' for 10G, 'Heavy Reading' for 25G, 'Dear Diary...' for 50G, 'Looks Like the Oddball' for 10G, 'Skull's Taken!' for 25G, and finally, 'Headhunter' for 50G. It's quite the investment, but in the end totally worth it.