Homebrew helps you install the UNIX tools that didn't come with OS X.
If you want to install Homebrew somewhere other than /usr/local see the installation instructions.
Look dude, I know what I'm doing!
Alright then… as long as you're sure:
cd /usr/local sudo chown -R $USER . curl -Lsf http://github.com/mxcl/homebrew/tarball/master | tar xz --strip 1
The wiki can answer your remaining questions.
Who Are You?
I'm Max Howell and I'm a splendid chap.