A blog is nothing more than an online journal with entries posted chronological from earliest to latest. That's the simple direct answer. A blog can be personal or it can be for a business. When it's personal it can be about anything you want from hoobies to jsut personal stuff you want others to read and know about.
A business blog is more specific and is related to the company, it's products or services, and/or the industry the company is in. It's great way brand a company, products or service.