A tongue-twister is a phrase that is designed to be difficult to articulate

Articulate sound

Articulate sounds are vocalizations that express the letters, syllables, etc, of any alphabet, or language. Non-humans cannot form articulate sounds, cannot articulate the sounds of their voice, excepting some few birds, as the parrot, magpie, Parakeet, etc....

 properly, and can be used as a type of spoken (or sung) word game

Word game

Word games and puzzles are spoken or board games often designed to test ability with language or to explore its properties.Word games are generally engaged as a source of entertainment, but have been found to serve an educational purpose as well...

. 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.

Tongue-twisters may rely on rapid alternation between similar but distinct phoneme

Phoneme

In a language or dialect, a phoneme is the smallest segmental unit of sound employed to form meaningful contrasts between utterances....

s (e.g., s [s] and sh [ʃ]), unfamiliar constructs in loanword

Loanword

A loanword is a word borrowed from a donor language and incorporated into a recipient language. By contrast, a calque or loan translation is a related concept where the meaning or idiom is borrowed rather than the lexical item itself. The word loanword is itself a calque of the German Lehnwort,...

s, or other features of a spoken language in order to be difficult to articulate.