Halo is a ring world. That is, its a "halo" built by some alien species that can be inhabited on the inner side of it. It is home to the Flood, a species of aliens that's sole purpose for their creation was to destroy all life.
The Covenant, a collection of alien races that have allied themselves, is in a war with the Human species. In Halo 1, the Covenant was in a major battle with the Humans and one ship, the Pillar of Autumn, escaped the battle and made a blind jump into deep space. When they exited their jump, they found themselves near Halo but lo and behold the Covenant had beaten them there (as their ships are faster).
To save the marines on board the Autumn, Captain Keyes manages to jettison the life pods to the ring world Halo and land the Autumn (sorta, crash is a better word) on Halo.
While on Halo, the Master Chief (you, a cyborg super warrior prototype) has to round up the scattered marines and make a stand against an entire regiment of Covenant military. In the process, the Covenenant awaken the Flood on Halo and set in motion a series of events that is to change the course of the galaxy forever.
Needless to say the Master Chief kills Covenant left and right, wacks flood as if they are flies (this is an understatement) and destroys the Halo ring world.
Not in time for it to send the message to the other Halo ring worlds scattered throughout the galaxy (approximately every 25,000 light years).
Halo 2 starts off with the Covenant finding Earth and invading. Dont want to get more into it than that, I dont really know any more but even if I did i dont wanna ruin the H2 story for anyone.
Its a first person shooter that, unlike many others, requires you to strategize how exactly you are going to get past the next area. So you got a pistol with 12 rounds left, 2 plasma grenades, 1 frag grenade and a plasma rifle half empty that you stole from one of those dead Covenant bastards. Up ahead, you have three Ghost's (light armored vehicles with great speed and maneuverability), a Banshee overhead (basically the fighter support), two Hunters (big hulking bruts with a nasty punch and a nastier RPG type energy weapon), 10 elite wort wort guys(complete with shields and plasma rifles and the ability to dodge your deftly placed grenades) and a whole slew of "Wake up, Cyborg!" guys (fodder for the Master Chief but a tiresome nuisance when there are bigger fish to fry), some of which are mounted on 360 degree rotating gun turrets. You figure it out