A family of Microsoft word processing software products for creating web, email, and print documents.
To add to what Charles has said, styles save time and let you create a structured document. With styles you can easily reuse formatting (no need to select text and repeatedly apply bold, different font sizes, etc. to create your heading) within a document and from one document to another.
Any document created in Word makes use of at least one style: the Normal paragraph style, usually.
You can revert to the formatting of paragraph styles in a document by selecting text and pressing Ctrl+SpaceBar (which clears font/character formatting not in style) and Ctrl+Q (which clears paragraph formatting not in style).