What does it mean that a programming language is a formal programming language? And which languages are formal programming languages? And which are informal programming languages?
I haven't found a good explanation yet.
Every programming language is a formal language, so that it does not make much sense to me to speak of a “formal programming language.” (Or does somebody know an informal programming language?)
Formal language is a language with mathematically precise construction rules. Or, more precisely, it’s a set of words over some alphabet. For example, if you take alphabet consisting of the letters a, b and c, a formal language over this alphabet could be a set {a, aa, aba, ca}. Of course such a language would not be very useful – the point is that with a decent set of construction rules you can create a language such as C or PostScript.
As for the “construction rules,” they could be a formal grammar (see grammar for CSS), a regular expression (see this gorgeous regex for e-mail addresses as defined by RFC 822), an automaton or a general algorithm.
If you feel you don’t understand the thing a tiny bit more, do not despair, for this is not a very good explanation :-) There’s a lot of thinking behind the concept of formal languages. I’d suggest that you try and digest the Wikipedia entry and look for more tutorials and books. It’s fun and you will learn a lot about the background of compilers, regular expressions, Turing machines and other wonders of digital mankind.