Gratis versus Libre
Which roughly translates to:
"for zero price" versuses "with little or no restriction"
Or to simplify it even further to a common analogy first used by Richard Stallman:
"Think free as in free speech, not free beer."
Should Joe Average the end user care if their software is Gratis or Libre? Whether or not a program is truly free doesn't affect the user user right?
Having a truly open software model ensures that the piece of software you love using today you will still be able to love just as much tomorrow. It means you don't have to worry about a good piece of software having it's code base get bought out. It also means that if a developer chooses to abandon a piece of software other developers are free to continue working on said piece of software.
In fact the only reason for keeping your "free" software free as in beer is to prevent your users from having choice. By not having open standards you can easily lock users into not being able to easily change from using your software later on.
Not all free software is created equal. Some authors believe more in liberty than others - so next time you download a piece of software that is "free" be sure to find out which kind of free it is!