Any set of words can be a phrase, for example: too expensive, ride a bicycle, after the lunch. Linguists define phrases as a group of words that are a single unit in a hierarchical structure, for example in a sentence.
Sentence is also a group of words, but they have to be linked together grammatically. Fore example: I like this dress very much, but it is too expensive. Every weekend I'm riding a bicycle. After the lunch we went to the cinema.