Language is what this whole wiki is about!
A language is a system of signals used by two beings to communicate with one another. Language is set apart from other form of communication chiefly by it's capacity for abstraction and reference to things not present. Languages are most commonly spoken or signed, with the vast majority of languages being spoken, and a few being expressed in some other manner.
Telling one language apart form another is usually quite simple, but there are cases where it can be difficult to say wheather or not two dialects belong to the same language, or if they are two different languages. This is because language, like living organisms, is subject to evolution, and therefore each language is analgogous to a species, and languages can in fact be classified genetically like living things can. Take for example English, English is a very large language, with very many dialects, sometimes two people who both speak english, but who speak different dialects, will not be able to understand one another, however, we can still call both dialects english, because there is a chain of dialects that can understand each other that lies between the two dialects, however, if for some reason everyone who spoke the dialects in between the two languages changed how they spoke enough that there was no longer a continum of understanding between the two dialects, they would then be different languages. Linguists think that this is how all languages came to be from thier parent language. In this example, we would have a new language that had english as a parent langauge.