Spyware and malware (malicious software) are nasty programs that someone sneaks onto your computer.  Malware is like a virus, but more directly criminal in intent.  What they do ranges from really annoying, like feeding you a steady stream of advertisements, to very dangerous, like stealing your account numbers, user names and/or passwords as you type them in, something called keylogging.  Malware can redirect your browser to a phishing site when you try to go to your bank site, or even let someone remotely hijack your computer.  Criminals create networks of hijacked PCs—botnets—to send spam or shop with stolen credit cards for example.