The aspect of grammar that refers to the order that the words of a sentence in a particular language are arranged in and how the different elements of a language's grammar interact with one another. Linguists contrast it with morphology, the study of how words are structured internally.
The complexity of a language's syntax varies and generally forms an inverse correlation with its morphological complexity. English, which lacks much in the way of inflectional morphology, has a great deal of syntax to make up for it, for example.
Here's a kind of introduction to syntax