A Chromebook is a portable "thin client" that runs the Google Chrome OS. Chrome uses the Linux kernel as its foundation and the Google Chrome Web browser as the overarching interface, through which you access files and run Chrome Web apps. Unlike traditional laptops, Chromebooks are designed for use while connected to the Internet. You can use some apps while offline, however; Google highlights them in its Chrome Web Store.
Chromebooks are optimized for Google apps, such as Gmail, Google Calendar, and Google Drive. You can install only Chrome Web Store apps on a Chromebook. Instead of, say, Microsoft Office, you'd create and edit Office-compatible documents using Google software and store your files in Google Drive.