Dropbox is a free file hosting service that allows users to store files in the cloud and access them from a variety of devices. Dropbox is available in both free and paid plans from its website, http://www.dropbox.com. The free plan, by default, comes with two gigabytes of free storage space, though users can earn up to eight gigabytes of free storage by completing actions on the Dropbox website or by referring other users to Dropbox. Paid plans offer more storage and are geared to be used more by teams rather than individuals.
Dropbox also allows file sharing between Dropbox users, using Shared Folders. Shared folders work sort of like an RSS feed - when one file is added, everyone else who is following the shared folder gets notified of it.
Dropbox has a host of applications available natively for a variety of devices, including Mac, PC, iOS, and Android. The desktop applications keep all of your private and shared files on Dropbox synced to your computer in their own Dropbox folder for easy offline access. The mobile apps allow you to access, upload, download, and share files with others, and all applications are available for free for users on all different types of storage plans.
If your question is about the web interface for Dropbox, it may already have an answer at Web Applications.
If your question is about the PC or Mac app, it may already have an answer at Super User.