it stands for malicious software, a.k.a. malicious advertising, malvertising
Slang for "malicious code," this term was primarily used by computer security programmers. For example, "Certain kinds of malware can be injected into a phone with the intent of destroying its functions." Regarding the Internet, this term is often heard in conjunction with hackers who use certain kinds of malware to infect computer systems and compromise the way they work. For example, "The hacker was arrested for setting up a botnet which sent out spam and bombard Web sites with massive amounts of data, thus shutting them down."
There is a long list of "-ware" terms in NetLingo.com; check out the "warez" definition to see a few of them.