Basically, they simply add more advertisements to about any webpage you go, and try to redirect you to more malware. You can use software like Ad Block to stop the ads from appearing in the first place, though it does not get rid of it all.
This type of malware is not that common, and are only usually found when bought from less popular locations. Hackers also can crack passwords in emails, games, bank accounts, and other things.
If the user pays this, they will be simply laughed at, as the program deletes itself [and the user would have paid at least ], or the ones who made the program will commit identity theft. Browser Hijackers are another common piece of adware, often these will install with Sponsored Downloads.
They exploit the fact that very few people properly read installers, and often allow programs to set themselves as your homepage, default browser and default search.
Some manufacturers disgustingly bundle Browser Hijackers such as My Web Search with their computers, and charge for removal.
The best way to solve OEM bloatware is to either run an anti-junkware tool or format. Only a few are deadly, which includes Win Fastener (which deletes a startup file to prevent the computer from booting) and Lose/Lose (which deletes personal files, and if it goes long enough, system files). Main article: Hacker Hackers aren't exactly considered malware, but are just as dangerous (if they have bad intentions) as some kinds of malware, as they can insert viruses into your computer, damage it via command prompt, or ddos you.