From their ‘About’ page:
Uncyclopedia is an encyclopedia full of misinformation and utter lies. You might say it puts the “psych!” in “encyclopedia”.
Further down the same page, in the ‘Who runs this place’ section:
The wiki was launched to great acclaim, particularly when the British discovered it and proceeded to fill it with humour, rather than humor. At which point the Americans didn’t understand it any more.