There's the software you install on your computer: that's generally not what we are talking about today.
Usually, “social software” is software installed on your web server (like WordPress or MediaWiki). We aren't really going to talk about these today, though blogs and wikis usually fall under “social software”
The third type of software runs on commercial or organization sites (like MySpace, etc.) This is what we are talking about today.
Flickr - almost certainly the best online photo management and sharing application in the world - has two main goals:
1. We want to help people make their photos available to the people who matter to them.
2. We want to enable new ways of organizing photos.