As I wrote earlier, my style may look odd but it is very simple, interesting and educating!
Let me first tell you "What is Marketing and Sales?".
Marketing - Driving people/potential customers to your shop
Sales - Selling your product/service to the people who've come to your shop
Ques: What is Online Marketing?
Ans: In the online world, shop = website, potential customers = website traffic/visitors. Thus, Online Marketing is "Driving visitors to your website".
Example: Amazon (http://www.amazon.com) drives visitors (i.e. poential customers) to its site and sell its products (especially books).
It is also called Internet Marketing or Web Marketing.
I hope its valuable. Your inputs are always welcome!