You still need to remember that the contents of Personal Package Archives are not checked or monitored. You install software from them at your own risk.
So, if you're using the most recent version of Ubuntu (or any version from Ubuntu 9.10 onwards), you can add a PPA to your system with a single line in your terminal.
sudo add-apt-repository ppa:user/ppa-name
Replace ppa:user/ppa-name with the PPA's location that you noted above.
Your system will now fetch the PPA's key. This enables your Ubuntu system to verify that the packages in the PPA have not been interfered with since they were built.
Now, as a one-off, you should tell your system to pull down the latest list of software from each archive it knows about, including the PPA you just added:
sudo apt-get update
Now you're ready to start installing software from the PPA like this:
sudo apt-get install software
where software is the program that you want to install.
Resources: launchpad, GPG/PGP Basics