internet marketing covers a huge range of topics... seo is part of internet marketing because it is a means to market a specific website to a specific target group of people.

In my best words I can describe internet marketing as either promoting/selling a product or service online and the tools and techniques that you use to succeed at it.

Sorry if that doesn't help much... it is a great question, and I may think of writing an article on it for my blog The One Year Millionaire Blog , it may already have some answers to your questions... feel free to take a peek