From a user's point of view, they seem equivalent. 
Some programs can be installed via `apt-get install` after adding a link to the `sources.list`. 

Others have to be installed by adding a ppa though command line, and as far as I can see, this method doesn't create an entry in the sources.list.

So there must be a technical difference? And, if at all, what is the difference between a PPA and a repository?