Yammer is an enterprise social networking tool. It was bought by Microsoft 4 years ago. Yammer is built around the lines of Facebook but it is built for the enterprise network.
It basically reduces email communication since everything can be done via Yammer. One can create different groups and list updates. Apart from that you can message anyone within the organization.
And another key feature is that you could also connect to external groups which is very useful. Overall it is a good collaboration tool to use within an enterprise.