Memories are collected by Sims in The Sims 2, and stored in a panel in the Sim's information box. Memories can be good or bad (determined by the Sims Aspiration). Bad memories can cause Sims to grieve. All memories, good or bad, can become topics of conversation, and Sims who hear about these memories also get an invisible "marker" of the memory, which means they can gossip to other Sims. Good memories are shown in green, and bad memories are shown in red.
Bon Voyage adds a sub-box to this called Vacation Mementos.
This feature is not included in The Sims 3, however moodlets act as temporary memories.