Git is a distributed version control system, like Mercurial.
At some stage, someone might put up a full tutorial showing how to use git, but for now, here's a badly written and terribly phrased guide on how to get and use a repository on redbrick's git hosting area.
Getting a git repository on Redbrick
Redbrick hosts git repositories for users. You (and any other users you want) can have write access to the repository. The world has read-only access via the web interface at http://git.redbrick.dcu.ie (although I'm not sure that they can actually pull from there - the system is still a bit of a work in progress), so don't use this to host any code you don't want the world to see. It's perfect for any small open source projects you feel like starting though, or if you want to branch/fork an existing open source project that's hosted on git.
Can I have a private repository that the world can't see?
Not on git.redbrick.dcu.ie. You could create one in your own home directory if you wanted to do this, but you'll need to figure out how to do it yourself :)
Can I use this to host my third year/fourth year/other important college project?
Short answer: No. And you shouldn't do this in your redbrick home directory either. There have been a number of times in the past where Redbrick has experienced problems meaning that a user who had the only copy (or the only backup copy) of a project weren't able to access it at some important deadline.
Long answer: If you have other copies of your code, and Redbrick isn't the primary (only) backup of it, and you just wanted to have yet another backup of it "just because", then maybe it might make sense to put it on redbrick. But I still wouldn't.
Creating an SSH key pair
Before you can use redbrick's git repositories from one of your computers (or from your redbrick shell), you need to create a public/private SSH keypair there if you haven't already. You'll need to do this for every device/place that you want to access git from. This might sound annoying, but it also lets you use git without having to type a password every time you need to clone/push/pull.
To do this on a Unixey (Linux/Mac OS X/on redbrick) system, run:
ssh-keygen -t rsa
You'll probably want to hit enter for most/all of the default options. Especially the first one (where to save the key). When it's done, you'll probably see something like this:
[you@your-computer ~]$ ssh-keygen -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/Users/you/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /Users/you/.ssh/id_rsa. Your public key has been saved in /Users/you/.ssh/id_rsa.pub. The key fingerprint is: 58:06:86:0c:83:5e:4e:ab:75:93:96:37:76:42:73:07 email@example.com The key's randomart image is: +--[ RSA 2048]----+ | .oo .o E. | |. +o. + . . | |. + . + = . | | . + * O . | | o o = S | | . | | | | | | | +-----------------+ [you@your-computer ~]$
(the little ascii art bit might not show up on older versions of SSH).
This has created two files on your computer - id_rsa and id_rsa.pub. id_rsa is your private SSH key, and id_rsa.pub is the corresponding public key. You keep id_rsa to yourself, and you can give id_rsa.pub to admins to allow you to access a repository. You can also upload it to a remote server (like redbrick) and put it in a specific location to allow you passwordless logins, but you can google for how to do that.
Ask the admins
Find an admin, and ask him/her/it if they'll add your repository to the system. They'll need the name of the repo, and any SSH public keys (not the private ones) belonging to devices that you want to be able to access your repository from sent to them. For example, if you want to be able to access the repository from your laptop, send them the id_rsa.pub file that you generated on your laptop (you'll find it in ~/.ssh). If you want to get at it from your desktop and your redbrick account, send them the id_rsa.pub files from your redbrick account and your desktop.
If you want to add more devices later, you can always ask the admins to add (or remove) some public keys.
Create the repository
Once the admins have done their work, you need to actually create the repository. You do this on your own computer (or redbrick account, or wherever one of your keys came from). If you asked the admins to create you a repository named "awesomeproject", you'd do the following:
mkdir awesomeproject cd awesomeproject git init git remote add origin firstname.lastname@example.org:awesomeproject.git
At this point you should do some work, like create a file and put something into it, or whatever. This is where a proper guide to git would be handy. Then commit your changes and do a first push:
git commit -a -m "My first commit" git push origin master:refs/heads/master
Things to look out for: When you're doing the "git remote add origin" command, make sure you add ".git" to the end of the repository name. The system that redbrick uses (called Gitosis, if anybody cares) to manage repositories needs that there. Also, in "email@example.com", the "git" user isn't a placeholder. You need use that user, not your own redbrick username.