Most simply, of course, "style" is a manner of doing something. As far as I'm concerned though, style has two distinct parts that are, I admit, somewhat incongruous.
The first part is perhaps the most obvious or general, and the easiest to apply in a broad variety of situations: I would argue that style most usually is not just a manner of doing something, but a distinct or individuating manner of doing something. This can be applied as a grouping method (eg, "he wrote in the style of beat poets") or singling ( eg "I recognized the style and immediately knew that my sister had written it"). While we're discussing style in reference to writing, this interpretation also applies to oh, just about everything else that's subjective: clothes, music, art, hair, couches, etc.
The second aspect I would argue is inherent to style is a sense of prescription or uniformity. Mostly, my initial reference here was the MLA style guide: though we all have our own unique "style" of writing, we all adhere to a set of rules. While we can easily see how "style" refers to individuating factors, I believe that it's just as important to acknowledge that "style" also references conforming guidelines. Inherent to "style" is a sense of both specificity and collectivity. this aspect, too, can be applied more generally, however, I imagine that it would be received on the whole with much more resistance than the first point.