Git:
before you say it, no I am not talking about the derogatory you might use when someone steps on your foot. Git is a low-level version control system engine that can be used for files and folders, it allows for branching within files/folder structures and allows for very fast read and write times.
Alright so why do we care? well one thing that is important for developers specifically working in open source software is for the work they are doing to remain up to date for everyone else that it might affect.
Imagine a scenario where Bob is working on a module that changes the gravity physics in an open source game. Jim is also working on the same game but he is working on a water flow effect, well water flow in this game will be affected by gravity, so if Bob makes changes to the module and Jim doesn't see them then he will likely need to rewrite at least some of his code.
On a low level version tracking makes perfect sense and git can be used on every operating system on the planet...to my knowledge generally it is used with a command line or terminal but there are graphical front ends for it as well.
GitHub:
Is likely besides steemit one of the most commonly visited websites in my browser history. Alright so what does github do? well they offer a git service on a cloud platform with access control rights, meaning anyone cannot willy nilly go and add code to the bitcoin repo (this means repository and is the term used when referring to a project) And yes I did say Bitcoin the complete work of the bitcoin repo can be found on github for anyone to look over and review or even copy if they so choose.
The primary benefit of using github is as long as the repo that you are working is set to public other people can come in and review your code, offering suggestions and finding bugs. This results in an ecosystem of open collaboration, requiring no permission or communication for the collaborative process to begin.
If you would like to read more about GitHub
Here are some interesting projects to check out on GitHub
Steemit (Front end) This is the codebase for the website.
Steem (Back end) This is the codebase for the blockchain.
Bitshares (Front end) A very close relative of steem, codebase for the website.