Adware is software that displays advertisements on your computer.  Adware displays advertising banners or pop-ups on your computer when you use an application.  This is not necessarily a bad thing. Such advertising can fund the development of useful software, which is then distributed free (for example, Android apps and browser toolbars, many of which are adware funded).

Adware becomes a problem if it:

installs itself on your computer without your consent
Installs itself in applications other than the one it came with and displays advertising when you use those applications
Hijacks your web browser in order to display more ads (see Browser hijacker)
Gathers data on your web browsing without your consent and sends it to others via the Internet (see Spyware)
Seem difficult to uninstall

Adware can slow down your PC. It can also slow down your Internet connection by downloading advertisements. Sometimes programming flaws in the adware can make your computer unstable. Some antivirus programs detect adware and report it as potentially unwanted applications. You can then either authorize the adware program or remove it from your computer.