B2Bridge

There are plenty of good definitions on Business Marketing all over the internet.

• Let's start off with Wikipedia: "Business marketing is the practice of organizations, including commercial businesses, governments and institutions, facilitating the sale of their products or services to other companies or organizations that in turn resell them, use them as components in products or services they offer, or use them to support their operations. Also known as industrial marketing, business marketing is also called business-to-business marketing, or b-to-b marketing, for short."
• Marketingterms keeps it short to "Business that sells products or provides services to other businesses."
• In their excellent book on Business Marketing, Dwyer & Tanner describe business marketing as "marketing products or services to other companies, government bodies, institutions (such as hospitals), and other organizations."

Finally, google provides a series of book references and web pages to learn more about business marketing.