Right... this again...

Reach- is before Halo C.E. (1 if you prefer) it is about the first covenant/spartan battles

for more try this, because I'm tired of explaining it... it's not just you guys I have others who are incredibly thick headed
http://halo.wikia.com/wiki/Halo:_Reach


ODST- between Halo 2 and 3 on Earth

more on ODST, this one I can explain better but am too lazy to care
http://halo.wikia.com/wiki/Halo_3:_ODST