Document sharing has typically been done via e-mail attachments, which makes it difficult to track versions and edits from multiple authors. With SharePoint, you can store documents for collaboration to provide access to up-to-date versions of documents, the ability to track previous versions, and centralized management. Co-authoring in SharePoint Server 2010 adds additional capabilities by allowing multiple users to work on a document, at any time, without interfering with each other's changes or locking other users out. This is default state for documents stored in SharePoint Server 2010. Another advantage for administrators is that co-authoring is managed by using the same tools and technologies that are already used to manage SharePoint.
To learn about co-authoring in SharePoint Server 2010, see the following articles: