Confluence is a wiki. You can use it to collaborate on writing and sharing content with your team. Your team can be as small or as big as you like. It could encompass the whole world. Your content can be as simple or as rich as you like. It can be just words, or you can add images, diagrams, activity streams, web service integration and more.
In a nutshell, a wiki is a piece of software that runs on a server. It publishes web pages that you can read via a web browser such as Mozilla Firefox, Google Chrome or Microsoft Internet Explorer. The thing that makes wiki pages different from other web pages is the edit button. Provided that you have the right permissions, you can edit the page, save your changes, and the updates will be visible to other people immediately.
Confluence provides extra functionality that transforms it into a social collaboration platform: