Trojans are malicious applications that pose themselves as legitimate software in order to trick users to install them. Once on the victim's machine, it may run any number of malicious process to steal vital information or inflict damage to other software.
Bandjammer is an exploit that targets band myspace pages. Once the band's myspace page is hacked, it creates an invisible background image that links to a dangerous site. After hijacking the victim's browser, it sends the user to a fake codec page.