BitBucket, like GitHub, is a web-based hosting service for uploading and sharing code. It runs on the Open Source version control system Git. Developers use version control systems to organize their projects, to update and change code and to store these modifications in a central repository. Others can contribute to that repository and download and upload changes.
Git is a command-line tool. It does not have it’s own user interface, but you can install one. But generally, you simply work with git by entering short commands into the terminal/command-line. BitBucket is a web page where developers store their projects and can network with the BitBucket community. Many Open Source projects are hosted on pages like BitBucket so everyone can contribute, report issues and leave comments. You can also create private repositories where just a few invited people have access.
Contrary to GitHub, BitBucket is completely free for up to 5 users in a team. You can create as many repositories as you want and only pay per extra user beyond the free ones. This is why BitBucket may be cheaper and better for small teams that want private repositories, while GitHub is popular among the Open Source community because you don’t pay per contributers but only for private repositories.