Diaspora is an open source and distributed social network. This means that user generated content doesn’t reside on central servers owned by a company, and thus users retain full ownership of and control over all their content. It’s described as a privacy-aware, personally controlled social network.
One of the main purposes of Diaspora is to provide an alternative to Facebook and Twitter that is completely transparent in its handling of user’s information and where privacy is taken seriously.
Here is a 3 part video tutorial to help you get started with Diaspora.