Dropbox is an online storage service that allows users to store and share files, photos, and videos across devices. It provides automatic synchronization so any changes made are reflected on all linked devices. To use Dropbox, a user signs up for an account on the Dropbox website, downloads the Dropbox app, and installs it on their computer. They can then upload files from their computer to their online Dropbox folder to access them from any device. Dropbox also allows users to create shared links and share folders with others to easily collaborate on files.