Why is my CPU usage so high when on the internet?

I switched from Internet Explorer to Google Chrome- which has done nothing for the problem. That was a suggestion I had gotten from somebody. Some days it can handle how high it is and run ok but today it's very high and running slow and jerky again. We upgraded to 7 mbps from 1.5 and that helps things run better but still not understanding why it runs so high and computer constantly hums. If you do have any advice please give step by step answers and speak as if I'm a child because I don't understand the high tech computer stuff! I've tried to figure it out online but I don't know what all the computer acronyms are that tech people use to describe it.