Yammer is Twitter for companies
Like Twitter, Yammer is a service where you can post (small) messages. Yammer is mainly used to indicate where you are working, furthermore there is the big difference that you can only connect to a network if you have the same e-mail domain (or you get an invitation to join). To use the service, you don't have to pay any money, but if you want to manage the network as a company you have to take a paid account. Yammer can be used on your desktop or phone via the website as well as via an app. You can post messages to anyone on the network, but you can also send private messages or like a message.
Yammer 101: Getting started [YouTube] GyxihTX-_3I [/youtube]
Introduction of Yammer at the Ministry of the State