Mechanics - punctuation, spelling, etc.
Usage - knowing which word to use when (they’re/there/their, you’re/your/yore, etc.)
Grammar - parts of speech, knowing about subjects & verbs and how they match up (they loves the movies.... ah, no they love the movies), etc.
Syntax & Style - most folks use this "s" for spelling, but I find usage is about spelling, so we'll stick with syntax and style