Thanks. In Linux I used aptitude quite often to clean old kernel versions and install other verified software, and it felt good to install software from a relatively safe location. So, does this only install things that are in the Mac App Store?
No. Homebrew is pretty much exclusively for installing GUI-less command line 'stuff' in OSX. Things like Nginx, mySQL, RVM, Golang, Bitcoind, etc. etc. Unlikely you'd even need it unless you're doing some kind of development or non-Apple-approved tinkerage.
No, homebrew has a different repository from the app store.