What is bidding?
Bidding in context to Google AdWords means the maximum cost per click (CPC) that you are willing to pay for a single click on your ad. A daily budget is maintained which controls and manages the amount you pay for a day.
What is a keyword?
A keyword is a word or a phrase that you identify as being relevant to your ad while putting it for online advertising. It can or cannot be a part of ad text and trigger it.
What is Quality Score?
It is an estimate of how Google expects the keyword to perform. It depends on three aspects which are shown in diagram.