A phrase is two or more words that go together. A clause is a part of a sentence containing a verb and a sentence is a complete grammatical unti containing a main verb and ending with punctuation like a period/full-stop, exclamation mark or question mark.
nice people (phrase)
They are nice people. (sentence)
The Browns, the nice people I met on holiday, are coming for dinner. (sentence, but here there are two clauses in it)