Chatter is a feature of Salesforce.com that provides a way for users to collaborate with other users within a Salesforce organization. It has a Facebook like interface that allows users to follow records and other users, share files, update statuses, like posts, and post questions or comments at specific records or users. It is marketed by Salesforce as a cornerstone of the social enterprise.
Questions related to using Chatter, developing for Chatter, administering Chatter, integrating with Chatter, or anything else about Chatter should have this tag.
The introductory paragraph in the Chatter Administrator Workbook has a very good description of Chatter:
Chatter provides a rich suite of features, including user profiles, feed updates, comments, groups, and feeds. You can use these features to add a collaborative and social dimension to your Force.com applications. Chatter also lets data records play a part in this collaboration—so people not only follow other people, but also data that's important to them. In effect, your applications can bring people and data closer to each other.
The Chatter section of the Salesforce help site has overviews and step-by-step instructions for many Chatter features.
For more technical information on Chatter see the Chatter developerforce site. It includes articles, code samples and recipes, webinars, and links to the Chatter Administrator Workbook and Chatter API reference.