Constructed languages are widely used in movies and literature, specially, in fantasy. Tolkien invented several ones, and a lot of writers and movie directors have included these thing in their works. Some of them are very famous, as the Klingons' language.
However, Tolkien was not the first person who invented a language. Perhaps, he was the first one that used a constructed language in a fantasy work, but other people had invented another languages. The goals of these old constructed languages were creating auxiliary languages for international communication. There are hundreds of these languages. One of the oldest is Johann Martin Schleyer's Volapük, that it is almost extincted. But now, I would like to talk about Esperanto, one of most successful constructed languages.
Esperanto was created by L. L. Zamenhof in 1887. It had so successful because is very easy to learn but it is easy express difficult concepts and it is easy to speak it in various styles. Although it is strongly agglutinative, but you can use it in a more isolating way. For instance, usually you will say:
Mia ŝlosileto (My small key)
But you could write
Mia eta ŝlosilo
Mia eta ilo por ŝlosi
All three sentences mean, literally "My small tool for closing doors using a key". Because Esperanto works by prefixes and suffixes and you construct your words using them. That means that, if you remember the prefixes and suffixes and you know some roots, you can derive hundred of concepts from that root.
For instance, if you know that 'bovo' means bull, you know immediately that bovino (-in suffix for female) means cow, that bovido (-id suffix for child of) means calf, that fibovo is a despicable bull (fi- prefix means 'despicable'), that bovejo means corral for bulls... Also, you can make more complex constructions... Bovinejo means corral only for cows, Gebovejo corral for bulls and cows. And you can construct this funny word: figebovetejo that means 'corral for bulls and cows which are small and despicable'. Or malsanulinejo that means 'hospital for women', literally 'place for not healthy female individuals'.
You can easily construct concepts very difficult if you are learning another language. For instance, although the word is similar to the Spanish one, I had to see the dictionary for know the word 'bovine'. In Esperanto is, simply, 'bova': if you put an 'a' after a root, you obtain the adjetive.
I also like to construct my own languages, but I have not enough patience for finishing them. Perhaps, I will talk here about Wâttûleg, a language that it is intended to be difficult to learn; an 'anti-esperanto'. Only for fun.