The Exchange

The Exchange is the official place where creators can share their custom content for The Sims 1, 2 and 3. It is a community feature made available by Electronic Arts. It is free to up- and download from The Exchange once you have registered as a user. There is not a single Exchange for the entire series. Each game has its own site, with its own Exchange, which operates in its own way, according to its own rules.

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

The Sims 3
Some creators do not appreciate their work being uploaded to The Exchange. They'd rather upload their work to communities such as ModTheSims. Pay sites in particular are imploring people not to redistribute their content on The Exchange as it is free to use, though there are no legal ramifications to doing so.

Patch
There is a known 'Error code 5' bug with the Sims 2 Exchange. A patch can be downloaded to fix the issue here: http://thesims2.ea.com/community/Sims2SSLPatcher.exe (0.2 Mb)