A tongue-twister is a phrase that is designed to be difficult to articulate properly, and can be used as a type of spoken (or sung) word game.
Some tongue-twisters produce results which are humorous (or humorously vulgar) when they are mispronounced, while others simply rely on the confusion and mistakes of the speaker for their amusement value.
There are many fun tongue-twisters out there, however "The sixth sick sheik's sixth sheep's sick" is listed by the Guinness Book of Records as the hardest tongue twister in the world. So if you can say it with no problem at all: CONGRATS! You have conquered the top tongue-twister in the English language!