Interesting question Surbhi. 

A ‘Style Guide’ is a set of style standards for writing and designing of documents in an organisation. Basically, the ground rules of writing which differ from one organisation to another. Some media houses follow British English and some prefer the American style. Some use “double quotation marks”, others use ‘single quotation marks’. A style guide keeps the grammar and spelling rules so that journalists can maintain a consistent style.

So you now know why you hear writers talk about style sheets. Keep asking ur queries.