Within the domain of syntax two levels should be distinguished: that of phrases and that of sentences.
The phrase can generally be defined as a combination of two or more words which is a grammatical unit but is not an analytical form of some word. The constituent elements of a phrase may belong to any part of speech. A word-combination can also be defined as a compound nominative unit of speech which is semantically global and articulated.
The difference between a phrase and a sentence is a fundamental one. A phrase is a means of naming some phenomena or processes, just as a word is. Each component of a phrase can undergo grammatical changes while a sentence is a unit with every word having its definite form. A change in the form of one or more words would produce a new sentence.
Grammar has to study the aspects of phrases which spring from the grammatical peculiarities of the words making up the phrase, and of the syntactical functions of the phrase as a whole, while lexicology has to deal with the lexical meaning of the words and their semantic groupings. For example from the grammatical point of view the two phrases read letters and invite friends are identical (the same pattern verb + noun indicating the object of the action).
Phrases can be divided according to their function in the sentence into:
(1) those which perform the function of one or more parts of the sentence (predicate, or predicate and object, or predicate and adverbial modifier, etc.)
(2) those which do not perform any such function but whose function is equivalent to that of a preposition, or conjunction, and which are equivalents of those parts of speech.