Because The Sims 3 is a large and complex simulation, performance mods are highly popular.

While the base game has a story progression system, many players use mods to make the town's inhabitants lead more active and interesting lives independently of the player's active household. 3. Custom Content (CC)

The Sims 3 requires a specific folder structure (usually a "Mods" folder with a "Packages" subfolder and a Resource.cfg file) to recognize custom content.

If there is an interest in enhancing the realism or variety of the game, understanding the different types of available mods is a great place to start. 1. Visual Enhancements and Lighting Mods