It's one of those annoying corporate buzzwords. Viral marketing is basically promoting something by word of mouth. You're far more likely to pay attention to something that comes from a friend or someone you know than large scale advertising. The way it works is a company sends a freebie or something of interest to a select group of people in the hopes that they will promote it by telling their friends about it. The most common example is getting a funny picture or video in your email that subtly promotes a product, generally when people find something funny they forward it to their friend, who pass it on to their friends etc. A very cheap and effective method of advertising.
Street teams are a similar idea, rather than doing all the promotion yourself. you get a small group of people to promotion for you at ground level, they create a local "buzz" and people tell other people etc.
Hey, I could be good at this marketing racket.. er, I mean profession. Any chance of a nice job and lovely big fat wads of cash, nice Mr. Warner Music person? awww please..
