Communities are like different spaces within your org where you can allow different users in without exposing all of your org's private data to them. You could make a Community for partners to talk to your employees, and another Community for your customers to join in, for example. There is some more info on Communities here: http://www.salesforce.com/chatter/features/communities.jsp
All requests to the Chatter REST API (and Connect in Apex) are handled in the context of a single community. If your org does not have communities turned on, you don't have to worry about it. All requests made to the Chatter REST API that don't include a community id (or to Connect in Apex with 'null' community id) are processed in the context of the internal default community. If a real community id is passed in then the request is processed in the context of that community. There are also Chatter REST API endpoints for browing communities.
Communities are like different spaces within your org where you can allow different users in without exposing all of your org's private data to them. You could make a Community for partners to talk to your employees, and another Community for your customers to join in, for example. There is some more info on Communities here: http://www.salesforce.com/chatter/features/communities.jsp
All requests to the Chatter REST API (and Connect in Apex) are handled in the context of a single community. If your org does not have communities turned on, you don't have to worry about it. All requests made to the Chatter REST API that don't include a community id (or to Connect in Apex with 'null' community id) are processed in the context of the internal default community. If a real community id is passed in then the request is processed in the context of that community. There are also Chatter REST API endpoints for browing communities.