Bitbucket is similar to Github, a revision control systems that use either the Mercurial or Git.

Github vs Bitbucket

Following are the features that Bitbucket provide with all the plan
1. Unlimited private repos
2. Dedicated support
3. Code reviews
4. Custom domains
5. JIRA integration

Bitbucket has an advantage over Github as it offers free accounts with an unlimited number of private repositories. These private repo can have up to five users in the case of free accounts and if you invite 3 users to join Bitbucket then you can add 3 more users which make the total of 8. By this Bitbucket gains an advantage over Github.

If you are a team of 8 people or less developing a project and want to make use of some private online repository, then, Bitbucket suits you.
And if team size increases then you can go for different plans. See list of plans

Git Clients:
You can download GIT client and use GIT Bash or Git GUI to perform GIT operations.

Alternative, you can use Atlassian SourceTree which is a free Git and Mercurial client for Windows.