Git
What is git?
Git is the most commonly used version control system. Git tracks the changes you make to files, so you have a record of what has been done, and you can revert to specific versions if needed. Git also makes collaboration easier, allowing changes by multiple people to all be merged into one source.
ssh
Using the SSH protocol, you can connect and authenticate to remote servers and services. With SSH keys, you can connect to GitHub without supplying your username and personal access token at each visit. You can also use an SSH key to sign commits. When you set up SSH, you will need to generate a new private SSH key and add it to the SSH agent. You must also add the public SSH key to your account on GitHub before you use the key to authenticate or sign commits. For more information, see "Generating a new SSH key and adding it to the ssh-agent" and "Adding a new SSH key to your GitHub account".
You can find in this video a clear explanation about the most commonly used git commands.