Skycow.US Minecraft is a special version of Minecraft that uses different user accounts. This was originally set up to keep random trolls off of the /f/lockdraw MC server. It also lets folks who don't want to buy the whole game play on Skycow.US specific servers for free.
Getting a Skycow.US Minecraft account
To use Skycow.US Minecraft, you'll need an account. This is completely separate from your regular Minecraft username and password (if you have one) and has to be created through an invite. Right now, there's no way to get an invite besides asking Admin on /f/lockdraw or by email at admin at skycow dot us.
Once you have the invite link you can choose a username and password for Skycow.US Minecraft. For clarity it's probably best to use the same name you go by on /f/lockdraw, but it's up to you. Note that invite links can only be used once and you'll have to ask Admin if you need to change your username or password later.
Setting up the client
After you've got your account set up, you can download the client. Versions for Windows, Mac OS, and Linux can be downloaded here. Like regular Minecraft it requires Java to run, so make sure you have a recent version from Sun's download page.