Chrome extensions have access to anything the browser has access to, in full. They can also read and write data from a private backend, and store data about a user privately. It’s a good alternative to web apps if the primary use case for an application is going to be in conjunction with an existing app.
You can modify behavior on any webpages you visit, and specify specific behaviors for specific websites. This is a great way to customize the user experience for apps like Gmail.
You can deploy Chrome extensions privately, for testing or private distribution, or publish them to the Chrome Extension Store. All Chrome extensions work by default on Firefox as well, as a bonus!