The terminals have a confusing story. I found an explanation on what it all means.
AUTHOR: SIRSMOKEAL0T
Planet Earth was the last planet to be investigated for sentient life by Librarian (a Forerunner). Upon investigation he did discover sentient life. He indexed them, and in his final hour he quickly constructed a portal to the Ark, and sent them there (Librarian had spent hundreds of years indexing every living being in the universe). After that was done, he buried the portal without any sign that it had ever been there, in order to hide it from the Flood. Finally, he terraformed the Earth, so that when the Flood was gone, and his fellow Forerunners completed the Great Journey (read: repopulating the universe with the indexed lifeforms), they could return to that Eden (as he called it himself). The Forerunners established a defensive line in space against the Flood, Librarian crossed this line when he went on his quest to index everything. The Flood just kept growing, spiraling outward, the most logical thing to do: expanding. Librarian's friend, 'Didact', was concerned about his safety, and kept begging him to come home (Ark), but Librarian remained strong and pursued his near-impossible quest to safe life. Eventually, Didact couldn't stand to remain idle any longer, and went after Librarian with a fleet. Feeling sympathetic for Librarian and his ideals, he sought for a plan to defeat the Flood without firing the Halos, so life would be spared and Librarian's quest could be ended, forcing him to come home. Didact then created an absolute super AI: Mendicant Bias. This AI would assist him greatly in battle against the Flood, the strategy was to lure the Flood masses into battle, while Mendicant Bias, with his super intelligence, would engage a mass of Flood forces, but also coordinate a sneak attack straight at the Flood core: the Gravemind. Unfortunately, this plan could never succeed, as Librarian pointed out the 'thing' would simply retract and muster it's forces, it would do everything for self-preservation. Didact then used this 'retraction behaviour' to buy time, so he could go and retrieve Librarian and take him home. But Mendicant Bias, the AI, betrayed him. He returned with the Flood, and destroyed the waiting rescue party, separating Didact from Librarian. Didact was then forced to retreat. Librarian then witnessed a disturbing behaviour from the Flood; they were no longer spiraling outwards. They were moving in a straight line. Headed for the Maginot Line (Forerunner defensive line), going straight for the Ark. In his betrayal, Mendicant Bias had told the Gravemind of the Forerunner's plans, which would be successful in eradicating the Flood. To counter Mendicant Bias, Didact quickly constructed 'Offensive Bias' to hold him off. [SirSmokeal0t's note: just shows you how amazing the Forerunners were] When the fleet had returned, Librarian remotely blew up their Keyships (ships that were able to gain access to the Ark through distortion of time and space), effectively locking him out from the Ark, but the Flood also. Librarian remained at the terraformed Earth, saying to Didact he had found a nice mountain where he could spend his final seconds, after that he had to stop communicating with Didact as the Flood had found every comm channel, and laughed at him. Begging him to fire the Array (Halos), Didact finally did. And he had to go on the Great Journey without his beloved friend. After the Great Journey, the Forerunners returned to Earth (the planet terraformed by Librarian), and remained there, giving birth to the humans that now populate it. They did not pass on their knowledge to them, because they felt it was their time to 'give in'. They had been an irresistible force in the universe for too long. As Didact said himself: "Nothing in the universe is unstoppable. As long as you push hard enough, everything gives". This no longer applied to the Forerunners, so it was their time to 'go'.
http://www.bungie.net/Stats/Halo3/Default.aspx?player=Shagnarok
I am just missing the Legendary/Heroic, Marathon man, and a couple online ones.
GT is Shagnarok.