Twitter is a microblogging tool that allows the user to create a short message using 140 characters – see the Twitter page for more information
Yammer is an enterprise social networking tool. Unlike Twitter which share message publically, Yammer is used for private communication within organizations or between organisational members and pre-designated groups. Users can start a conversation, create a poll, share news, links, files and more as well to view and reply to posts from coworkers across your organisation.
Social Media for Learning 