A constructed language (conlang in short) is a language that has been consciously devised in entirety or in vast majority by an individual or a group. Contrary to natural languages, it has not evolved naturally.
Some conlangs are created to make communications easier (auxiliary languages), others are to further flesh out a fictional world (fictional languages), some are just linguistic experiments.
The most famous of conlangs is undoubtedly Esperanto.
Fictional languages are a subset of constructed languages – they usually exist in a fictional world of a book or a film. They usually have features rare in human languages, but can usually be spoken by the humans – there was an experiment to bring a child up as a speaker of Klingon.
A/N: The links lead to Wikipedia pages.
A/N: This link leads to the best dictionary. I wonder why doesn’t it have a Wikipedia page? It’s about as well-known as Dothraki…