Atlas is a web-based mapping tool that allows users to create custom online maps. The document provides step-by-step instructions on how to use Atlas, including how to create an account, add locations to a map, and attach additional information like links, photos, and directions. Key features of Atlas highlighted are creating maps quickly, embedding maps on websites, and allowing readers to comment and add photos to locations. The tutorial walks through adding a user's hometown and other places they have visited to a personal map as an example.