Yes, PPA's are personal software repositories that tend to carry things that aren't available in the official Ubuntu repositories. These kinds of things range from apps not available anywhere else, software that is more up-to-date than what's available in the official repositories such as GIMP, LibreOffice, etc., themes, and tweaks to things that aren't provided for by ubuntu itself.
While the repositories are officially "untested" by Ubuntu, I have yet to use one that had dangerous code.
Desktop:Mint 17.3 Cinnamon
Laptop: Mint 17.3 MATE