Syntax, originating from the Greek words συν (syn, meaning "co-" or "together") and τάξις (táxis, meaning "sequence, order, arrangement"), can in linguistics be described as the study of the rules, or "patterned relations" that govern the way the words in a sentence come together. It concerns how different words (which, going back to Dionysios Thrax, are categorized as nouns, adjectives, verbs, etc.) are combined into clauses, which, in turn, are combined into sentences.
For further information on Syntax, please see Wikipedia.
Wikipedia's Syntax article.