Yammer (yammer.com) is best summed up as "Twitter for business".
Yammer takes the basic concept of microblogging (Twittering), and added business-friendly features like a private network, message replies, and network security.
To get a good sense of what Yammer can do for your company, watch this 7 minute demo of Yammer by founder David Sacks when they launched (September 2008).
The heart of Yammer is the dashboard. On top, there is a box asking you the question "What are you working on?". Below is the stream of responses from other people in your company.
1. They're both microblogging services. The basic premise of a message is simple. For Twitter, it's answering the question "What are you doing now?" in 140 characters or less. Yammer asks, "What are you working on?".
2. Messages displayed in a stream. The newest messages appear at the top of a single company-wide stream of consciousness.
3. You can follow specific individuals. Or read all messages from all people.
4. Multiple input points. You can post a message via a webpage, desktop widget, iPhone app, Blackberry, IM (AIM, Yahoo messanger, MSN, GTalk), or SMS (text message).
1. Yammer groups require all users to have company email addresses. For example, there's only one About.com group, and you must have an email that ends in @about.com to be a member.
2. Messages are not limited to 140 characters. You can even attach files.
3. You can reply to individual messages. You can then view messages in a threaded stream where replies are hidden beneath the top-level message.
4. You can tag a message by adding a # to the front of a word. Messages can be easily filtered by tags, and you can see which tags are popular.
Yammer is free. There is a paid version that adds "admin" privileges. The admin plan costs $1 per user per month.
It gives you extra security like IP address filtering), SSL encrypted traffic, and limited Yammer employee access to your messages. Admin powers also include management features like message deletion, user deletion/banning, and customized styling.
A direct competitor to Yammer is Present.ly. Membership is also limited to company email addresses only.
Twingr lets you create private microblogging communities without the restriction of having a company email address.