SharePoint is basically a Content Management System from Microsoft, today known as MOSS - Microsoft office SharePoint server.
MOSS is often used for collaboration, file sharing and web publishing. It allows organizations to create document sharing workspace with features as Authentication, File versioning, Check in-Check out etc.
MOSS is build upon ASP.Net 2.0 platform hence its highly customizable too. To get idea of how SharePoint helps in collaboration see below video.