The Collector Base is located in a debris field in the galactic core, and can only be reached by travelling through the Omega 4 Relay, only possible with a Reaper IFF transponder.
The final mission of Mass Effect 2 takes place in the Collector Base. It is thought to be a suicide mission. The team must rescue the crew of the Normandy, then proceed and destroy the base. Throughout the mission, teammates are assigned to various roles; assigning teammates badly can lead to their death. However, it is possible to escape without a single casualty.
After Shepard has defeated the Human Reaper who serves as the game's final boss, the game's final decision is made: whether to destroy the base, or simply kill all of the Collectors and hand the base to Cerberus.