You should take a look at the Wikipedia article, but the gist of it is that git is a piece of software that allows many people to work on the same code and keep track of each other's changes.
Have you ever tried to work a single project with multiple people? It can very quickly become a mess. If I'm writing one part of a program on my laptop, and you're writing another part on yours, what happens when both of us finish? Now I have to send you my code (or vice versa) so that you can integrate my changes into the program. Then you have to take my copy of the program, compare it to your copy of the program, and manually edit in my changes.
What a lot of repetitious work! Isn't that what we have computers for? Yes! Software that tries to solve these kinds of problems is generally called version control, and git is one of the more popular (and powerful) version control systems currently in use.
Go to git-for-windows.github.io, and click "Download". That should take you to a page with some release notes on it. Scroll down the section marked "Downloads" and find the appropriate .EXE file file for your machine. This will probably be the file marked "Git-2.14.2.3-64-bit.exe". If you're lost, here are some direct 64-bit and 32-bit download links.
Once you've downloaded the installer, follow the on-screen instructions to finish the installation. If you're not sure what to do, accepting the defaults should yield reasonable results.
If you already have XCode installed you needn't do anything more here. XCode includes git by default.
If you don't have XCode and would like to have git, download git-osx-installer from SourceForge. If you follow the SourceForge link, you should be presented with a list of various versions of git-osx-installer. Most likely you'll want the latest one ("git-2.14.1-intel-universal-mavericks.dmg"), but this may depend on your operating system version.
Search for "git" in your distribution's package manager, and install it.
When you want to download and work on someone else's code, the first thing that you want to do is download the person's source code and associated change history from their repository (a.k.a. their repo).
Let's try this out; Open your operating system's terminal application, type the following and press [ENTER]:
$ git clone https://github.com/FoothillCSClub/git-workshop.git
This will clone the source code for this workshop into the folder
"git-workshop". (The $ symbol represents whatever your operating
system gives you in the way of a prompt. Don't type it.)
You usually want to move into a folder after you've just cloned it:
$ cd git-workshop
Open your favorite text editor and write a few lines about yourself.
Don't say anything too personal, because we're going to share this with
everyone else in a few steps. Save the file as ${your_name}.txt, where
${your_name} is your first name. Make sure to save the file in the
git-workshop folder.
Now that you've made your changes to the repository it's time to publish them so that the rest of us can use them. The first thing to do is to commit your changes to your local copy of the repository. Committing tells git exactly which changes you want to publish, and it also allows you to provide a message attached to your changes to inform your fellow programmers of why you made the change.
To commit your changes type:
$ git add ${your_name}.txt
$ git commit -m "${commit_message}"
Where ${commit_message} is your comment for this commit. In this case,
it should probably be something like "Introducing myself", but feel free
to be creative.
If you type
$ git status
You should see something like the following:
On branch master
Your branch is ahead of 'origin/master' by 1 commit.
(use "git push" to publish your local commits)
nothing to commit, working tree clean
This means that you have a some changes that you've made that only you can see, because you haven't sent them to anyone else's repo. To fix this problem type:
$ git push
This will contact CS Club's server and upload your changes, so that the
next time anyone does a git pull in their own repo, they will download
your changes, an incorporate them into their source tree.
Congrats! You've made your first use of git!