First off ISACCCCCCCCC ( I don't really care how many stupid c's are in your name), you should've had a spoiler alert since I haven't played number 3 yet...fool.
Second, ODST is a follow-up to Halo 3 but it actually takes place before Halo 3. You play as an Orbital Drop Shock Trooper (hence the acronym) and you are more or less protecting New Mombasa from the Covenant and Flood and all that stuff. The only differences are the plot (again, it fleshes out the story of Halo 3) and that you do not play as Master Chief- you don't have rechargeable shields, super speed or high jumping ability. Think Rainbow Six without teammates and you're on the right track. It's more strategic and cautious than the run-and-gun nature of the other Halo games.
I really do want to get it though- it'd be interesting to see how this shapes out.