The garbage collector groups objects by age and collects younger objects more often than older objects.
New objects are created in generation 0. As most objects are short-lived, only a small percentage of young objects are likely to survive their first collection. Once an object survives the first garbage collection, it gets promoted to generation 1.
All objects in generation 1 that survive collection are promoted to generation 2. Surviving objects in generation 2 stay in generation 2.
Also, there is "Large Object Heap" for objects bigger than 85,000 bytes. That generation is not compacted.
To learn more about the generational garbage collector, refer to MSDN.
All tabs representing live objects have a "Generations" view:
To see the generation of a single object, use Quick Info: