| We control what|
you think with
|Said and done|
|Jargon, buzzwords, slogans|
Volapük, also known as Volapꞟk is a constructed international auxiliary language devised circa 1879 but not fully published until 1880 by Johann Martin Schleyer, a Roman Catholic priest in the Grand Duchy of Baden in Germany. Schleyer felt that God had told him in a dream to create an international language.
Comparison to Esperanto
If you think Esperanto is bad, then you will hate Volapük. Instead of using the extremely common AEIOU 5 vowel system, it uses the much rarer eight-vowel system of North Germanic/Uralic languages. It has contrast between a, ä, and e (English technically has more than five vowels, but their subtleties are assumed and not codified).
Its other three vowels are represented by the letters Ä, Ö and Ü, although these can also be replaced by the diagraphs AE, OE, and UE respectively, or with original letters invented by Schleyer (Ꞛ, Ꞝ and Ꞟ respectively).
Its vocabulary consists of English, German, and French words deformed beyond recognition. For example, the name "Volapük" itself is derived from the words "vola" and "pük", derived from English words "world" and "speak", respectively. Also, the accent is always on the last syllable of each word, so the name of the language should be pronounced "volaPÜK".
Volapük has four grammatical cases (versus two in Esperanto), adjectives have agreement rules more complicated than in Esperanto, and verbs have complex conjugations.
In short, Volapük is basically a garbled and unharmonious mishmash of Germanic, Uralic and Altaic. Your mileage may vary, of course, but Esperanto is generally agreed to feel more aesthetic to the ear (and to the brain).
Volapük was one of the first serious attempts at creating an international language, and it came before Esperanto. Volapük conventions took place in 1884 (Friedrichshafen), 1887 (Munich) and 1889 (Paris). At one point, a community of millions of speakers existed. Volapük, however, was displaced, for several reasons by 20th century. Schleyer wanted complete control over the growth of his language, and would become very upset when, for example, people attempted to create new words for new situations, leading to conflict between Schleyer and other prominent speakers. In addition, Esperanto was found to be far simpler than Volapük. Volapük today only has between 20 and 30 speakers, and it is mainly kept alive through the internet. 
The Dutchman Arie de Jong briefly revised and revived Volapük after World War I, for example adding the sounds /r/ and /w/, which were not originally included in Volapük for the benefit of speakers of Chinese and Russian, respectively, mangling words like "rain" into lömib (rein in the revised version).
Surprisingly, there is a Volapük edition of Wikipedia and even more surprising is that it has over 124,000 articles. However, this is because back in 2007, a Volapük fan created a bot to automatically create articles on the Volapük Wikipedia. Most of these articles are very short articles on the topic of geographical locations. An editor proposed that the Volapük Wikipedia be closed from editing, but the proposal was declined. . Several other Wikipedia editions in obscure languages have faced similar problems.
The word "Volapük" in other languages
In many languages (including, amusingly, Esperanto itself), Volapük (or a variation of it) often means something like "gibberish". In Russian, "волапюк" (volapyuk) refers to a system invented for using early computers with Roman-alphabet keyboards to write a semblance of Cyrillic text. The word волапюк (or воляпюк) looks and sounds funny to Russian speakers, so it was a natural choice for naming such a cobbled-together system. (See the Wikipedia article on Volapuk encoding.)