Grammar is prescriptive (actual rules) . Syntax is descriptive and refers to the rules and structures of a language.

To be honest, they mean the exact same thing, though they have slightly different flavors and feeling to them. "Grammar" is more basic and rudimentary-sounding on like a grade school level, while the subtleties and mood coming from "syntax" when used in English is like that of a very analytical, collegiate sort. I hope that clarifies things a bit.