The Exchange

The Exchange is the official place where creators can share their custom content for The Sims 3. It is a community feature made available by Electronic Arts. It is free to up- and download from The Exchange once the player registers as a user. There is not a single Exchange for the entire series. The Sims and The Sims 2 each had its own site, with its own Exchange, which operated in its own way, according to its own rules.

The Sims
The Exchange for The Sims had sections for families, as well as for lots for the various community areas. In general, downloads from this Exchange did not include any custom content used on the lots or any custom skins worn by the Sims in a family.

The Sims 2
The exchange for The Sims 2 (along with the entire website) went down sometime in the Spring of 2011. After complaints, however, the site returned, with the content from the exchange slowly being added back. On January 14, 2013, The Sims 2 Exchange was removed by EA, along with the rest of TheSims2.com.

Patch
There was a known 'Error code 5' bug with The Sims 2 Exchange. A patch to fix the issue could be downloaded here: http://thesims2.ea.com/community/Sims2SSLPatcher.exe (dead link).

The Sims 3
The exchange for The Sims 3 is split into 14 sections, accessories, clothing, hair color, households, lots, objects, patterns ( from Create a Pattern ), pets, pet accessories, pet colors, Sims, tattoos, stories, and worlds ( from Create a World )

While playing the Sims 3, players can directly upload houses, and other objects straight to the exchange through the Launcher.