Copywriting is the act of writing copy for the purpose of selling or marketing a product, business, or idea. The term copywriting is often used interchangeably with content writing or freelance writing, though this is technically incorrect. A copywriter is one who writes for promotional purposes, and although they may very well be a freelance writer, they have a specific marketing purpose.
A copywriter might write advertising copy, sales copy, sales letters, commercial scripts, or other types of marketing media for a business, such as brochures, billboards, or mass mailings.