According to Wikipedia, The Free Encyclopedia, a tongue-twister is a phrase that is designed to be difficult to articulate properly. Tongue-twisters may rely on similar but distinct phonemes (e.g., s [s] and sh [ʃ]), unfamiliar constructs in loanwords, or other features of a language.
The hardest tongue-twister in the English language (according to Guinness World Records) is supposedly The sixth sick sheikh's sixth sheep's sick.
Below is an example of a long tongue-twister:
Betty Botter bought a bit of butter.
The butter Betty Botter bought was a bit bitter
And made her batter bitter.
But a bit of better butter
Makes batter better.
So Betty Botter bought a bit of better butter,
Making Betty Botter's bitter batter better.
If you want to know more examples of tongue-twisters, just visit Let's Learn English.