if your into the halo series then I would strongly suggest getting the full game for halo 1, 2 , 3 and ODST. you will get a lot more out if it if you do and it wont be as confusing
but onto your question, halo ODST takes place after halo 2 and before halo 3
but halo ODST was released after halo 3 so ODST just really fills some storyline gaps
ODST stands for orbital drop shock trooper and that's what you play as instead of the conventional master chief
because you are only human in ODST you cant jump as high as well as some other realistic limitations
alot of people complained about the play time in ODST with some people telling me they completed it in 3 hours or less whereas the halo 3 game is lot bigger. in my opinion i would go with halo 3