Syntax is the traditional name in the study of grammar for the branch of the subject which deals with the relationships between words, their arrangement, the construction of sentences, the rules of word order and so on. Much of syntax is covered in AWE's grammar course by the section called structure of a clause.
The traditional name for the other principal branch of grammar is accidence, which deals with inflections, which are changes in the forms of words to reflect different grammatical meanings, or positions in various syntactical constructions. Accidence is not of huge importance in English - it is a more important feature of the languages which began the study of grammar in Europe, Greek and Latin. Only one verb in English has more than five forms ('to be'): a Latin verb may have over 144.
Syntax is a technical term in other subjects than language. These are not covered here.