A formal language can be entirely defined by the way a set of symbols is organized. This way, it can be defined before it has any meaning- without any reference to any meanings of its expressions and before any interpretations are given to it. Formal languages are tools in the field of mathematical logic and computer science. As such they are designed to express the logic of executable programs.
Like languages in linguistics, formal languages generally have two aspects: syntax and semantics. The syntax of a language is what the language looks like without regard to its meaning. The syntax is defined by the set of rules that govern how the symbols of a formal language can be used to construct valid expressions. The semantics is what those expressions mean. This is formalized in various ways, depending on the type of language in question.
A formal language can be identified by its well formed formulas (wffs). The set of well-formed formula of a particular formal language is determined by a fiat of its creator. Usually, the determination of what is and is not a well-formed formula is laid down by designating a) a set of symbols which are the alphabet of the language and b) a set of formation rules which determine what sequences of symbols from the alphabet are the well-formed formula of the language.
The branch of mathematics and computer science which studies exclusively the theory of language syntax is known as formal language theory. In formal language theory, a language is nothing more than its syntax; questions of semantics are not addressed in this specialty.
Reference:
http://en.wikipedia.org/wiki/Formal_language
