VJ is the short form for "video jockey" like Keaton said. (Similar to DJ which stands for "disc jockey.")
It all goes back to how rap music started. Back in days of yore (i.e. the 1970s), New York discotheques ("discos" for short) were clubs playing music for dancing. These disco clubs hired DJs - or disc jockeys - to play records. A good DJ could keep the dancefloor filled all night. A great DJ, as the years progressed, could "beat-mix" or blend songs together to set up the mood for the evening. As it became popular for DJs to talk between records, alternately announcing who the record was by and then encouraging the disco-goers' revelries ("Let me see you put your hands in the air for the new song by Abba"), these disco-hosts became gradually known as MCs ("Master of Ceremonies", like the host of a formal dinner party). These MC-DJs became more and more verbal in their inter-song banter, and by the 1980s whole songs were recorded featuring what became known as "rapping". Those who didn't want to be referred to as "rappers" were described as MCs or "emcees" in a play on the widely-used "deejay" spelling of DJ. The widely-recognised debut of rap music was "White Lines" by Grandmaster Flash, though many cite Kool Herc in its development. Whilst "rap music" quickly became predominantly the preserve of disaffected working class black Americans, as with all musical forms no one culture or racial group could wholly claim ownership of the genre. In the mid-to-late 1980s hybrid forms and collaborations emerged, including the seminal reworking of Aerosmith's "Walk This Way", which featured popular rap combo Run DMC. Other examples include the later Anthrax-Public Enemy hit "Bring The Noise" and Faith No More/Boo Yaah Tribe's "Another Body Murdered". So striking and popular were these genre-splicing records that an entire new genre developed around these rap-rock hybrids - it became known as "Nu Metal" - a cleaner, pared-down, solo-free variation on Heavy Metal and Rap. Enter Linkin Park. Of course, each new style of music only really lasts for a very short time, and Nu Metal per se is effectively dead. Mind you, just as Foo Fighters outlived grunge, the evidence of six million album sales would suggest that Linkin Park aren't going anywhere just yet.