|The Structure of English Language Syntax|
Parts of speech, or word categories, indicate what words usually do, or may be expected to do. Some of these categories - such as nouns and pronouns - make sense when we consider words in isolation. Others - such as conjunctions or prepositions - only make sense within a longer structure, a phrase, clause or sentence.
Note that these three terms are traditional, and do not easily describe how strings of language work. All are broad categories. The sentence, especially, is much more characteristic of written than of spoken English, and of formal rather than informal usage. Alternatively, we may say that spoken English contains sentence types not usually found in writing.
The internal grammar of phrases, clauses and sentences refers to the principles (sometimes mistakenly called rules) of structure and organization. Be aware of the tension between model structures devised for textbooks and guides for learners of the language, and the syntax of real sentences (those you have found in speech or writing), which you are subjecting to analysis.
A phrase is a useful all-purpose name for any short sequence of words (or even a single word, considered as an element in the structure of a clause or sentence), especially a grouping which could be replaced by a single word. A phrase which works like, or equates to, a noun is a noun phrase, one which qualifies a verb is an adverb phrase and so on.
A clause may be short or long, but must contain at least one main, finite verb. A short clause may in fact be identical with a verb phrase: the two terms reflect differences of emphasis or analysis in regard to the language string in question.
Phrases are especially important for analysing spoken data, and some kinds of written text (such as advertisements or information leaflets) where (written) sentence forms are not considered essential.