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Go to github.com. It has a tutorial about git, repositories, pull requests and etc.
Repository is open source project that can anyone see it, use it and recommend changes. Also it can be used when working in teams.
Git is a source code control or revision control system that stores a project, or a set of files, as they change over time. Git stores the information in a data structure called a repository. The repository does not have to be open source. Repositories can be hosted on a local machine or on a public or private server. GitHub https://github.com is a web based Git repository and internet hosting service. GitHub Inc. is a commercial enterprise that charges for private repositories; public and open source projects are free. Think of a repository as a database, or filing cupboard, that not only has a copy of your files but also a copy of every version of your files. So if you've changed things and it goes wrong you can go back to the version you had yesterday, last week , or last year (providing you saved it in the repository). Revision control repositories are not just used by programmers. They can be used to keep any form of data but are idealy suited for textual data.
Don't confuse GitHub with Git itself though. GitHub is a service provider for Git repositories. Git itself is a free, open source source code management tool. To learn more about Git see https://git-scm.com/ where you will find lots of documentation.