Adware is simply software that displays popup ads or other ad related screens in your web browser or on your desktop. It arrives on your pc without your consent it has no easy way to uninstall or intentionally reinstalls if you remove parts of it, changes your pc and browser settings without your consent, and can even communicate with external servers to study your browsing habits. This can also be spyware since they are very close in what they do, although adware is strictly tracking and displaying ads, while spyware can alter alot of things on your pc. The main way you get adware is by visiting suspect sites (like porn sites) where the sites are setup to draw alot of traffic, but then they use browser security holes to force your browser to automatically download and install their software. Activex controls through IE have been known to have alot of security holes, as have java in browsers.

Downloading email attachments is another way adware can arrive, they use this method the same way virus writers used it so often in the past, although this method is rather remote today compared to other methods.