What is it?
Yammer is an enterprise social network. It has the ease of use of something like Facebook or Twitter, but is focused on collaboration, file sharing, knowledge exchange and team efficiency. It is delivered as Software as a Service (SaaS) meaning it keeps changing and pushing out new features.
Why is it DIY?
Anybody can start a group or an external network in Yammer without needing to get approval from anybody higher up. Yammer also connects people who haven’t been in touch with eachother before and can cut across the business. This makes Yammer a meritocratic rather than a hierarchical tool.