Yammer comes integrated into your Dynamics CRM instance. You just need to configure the connection to get it running.
Yammer is the enterprise social network. The company was created in 2008 and bought by Microsoft in 2012. The social network is a corporate-friendly social media network. Any user can join the network with a business email account, as long as the company's domain is registered. Once joined, you can create internal and external networks for employees, customers, suppliers, and so on.
The application structure is quite similar to Facebook when accessed online, outside of Dynamics CRM. You have a newsfeed on the main page, and the ability to follow users and send and receive private messages.