A style guide covers text and imagery. It includes preferred spelling, grammar, and punctuation conventions to follow. It may address regional variations in a language, and how to present numbers, prices, dates, etc.
Style guides provide consistency among different content items.
A style guide incorporates voice and tone guidelines. It indicates preferred terminology: which words to use, which spelling to use, and any jargon to avoid. Style guides can be incorporated into software used to create content.