What is Adware?

Adware or advertising-supported software is any software application in which advertisements are displayed while the program is running. These applications include additional code that displays the ads in pop-up windows or through a bar that appears on a computer screen. Adware helps recover programming development costs, and helps to hold down the price of the application for the user (even making it free of charge)—and, of course, it can give programmers a profit, which helps to motivate them to write, maintain, and upgrade valuable software.
That could also cost a lot of CPU-time and also your nervs! :(

Some adware is also shareware, in that users are given the option to pay for a "registered" or "licensed" copy, which typically does away with the advertisements.
But the most are such you never want at all!