Halo Reach is the last game in the Halo universe.
Halo Reach is the start of the whole Halo universe and starts before the events in Halo: Combat Evolved. You are a member in Noble Team who is willing to fight for Reach and humanity in the campaign mode.
Campaign in Halo Reach has to be one of the best campaigns you will ever play and one of the best in all Halo games. Everything in the campaign is all mixed up with big fights and going covert to snipe some alien butts. But I have to say it is definitely worth to play through the campaign. Even you can play up to 4 player co-op even on your xbox or online.
Multiplayer is so much better with a better ranking system and you gain credits to buy new armour for your spartan.
Graphics have been improved since Halo 3 and Halo Reach is so beautiful. The graphics are definitely very detailed and also the weapons are very detailed as well.
There are other modes like Forge which is so much better than Forge in halo 3 and you make some incredible levels on each multiplayer level. Even some of the bungie creators some created 5 forge levels on forge world which is the biggest level ever created by the bungie team. Theatre mode makes a return and custom games and also the awesome firefight mode from ODST is back.
Halo Reach is definitely worth a try especially for halo fans.