Homebrew is a package manager that installs the stuff you need that Apple or Linux didn’t.

Homebrew installs packages to their own directory and then symlinks their files into /usr/local.

Homebrew won’t install files outside its prefix, and you can place a Homebrew installation wherever you like.

It’s all Git and Ruby underneath, so hack away with the knowledge that you can easily revert your modifications and merge upstream updates.

There is a ton of documentation on the developer's website.

Donations accepted here.