Some common terms you'll hear.
Adds new files or changes to files. Nothing gets stored, it's on the dock, ready for the git commit command to send it on its way.
A break-off of your project. It's in your repository, but changes there don't affect your master branch. Say you want to test something, you create a branch, make and test your changes. Once you are sure everything is OK, merge into master. If you mess something up, you're on a branch, the master is fine.
A copy of a repository that is on your local machine not on a server elsewhere. This is where you mess around.
A change/revision to a file/group of files. You want to add a message as to what was changed.
Difference between two commits. Normally shows what is added/removed since the last commit.
A copy of someone else's repository that will be in your account. Let's you mess with it and not mess with theirs. Think you have something worthwhile, you can submit a pull request and see if the author will update it with your changes. Also let's you stay recent by pulling original again.
When the changes from one branch are applied to another. Usually a pull request.
When you pull in changes and merge them. Changes have been made, you'll want to pull those in.
These are the proposed changes to a repo. Can either be accepted or rejected.
Pushing the changes you've committed to a remote repository... GitHub. You make a change on your clone, you'll have to push those changes so others can access them.
Version that is on the remote server, GitHub. This is what gets cloned and your changes are merged here.
The folder of everything. It holds all of your files and each revision of the files.
This working version, the main copy on the original repository. It's upstream as that is where all other changes come from. You are working on a downstream form or branch.