Screen printing was invented in China around the year 1000 AD. It wasn't until around the 1960's that screen printing was widely recognized as an art form. You could say that the "Rock Gig" posters are what really shed some light onto this underground art. They were made famous by artists such as Andy Warhol and Roy Lichtenstein. Today it is now known as one of the most creative and effective ways of advertisement. Using methods such as typography, illustration, graphic arts, and paint; artists will present information in a way that draws the eye in and conveys the message or event details in a much more unique way than traditional advertising such as magazine ads, newspaper ads, commercials, and word of mouth.
Being a screen printer myself, I happen to love these posters because I know the thought process that is behind the design. In most cases, designers will choose no more than 5 colors to generate their piece of art. But enough talk about them, just take a look at some of these great examples. And look for a new post which will go in depth on screen printing and the techniques used for every step of the process.
And 1 side note with my personal opinion. I have found that the bands who take the time to create these posters are some of the best out there. It shows that they respect art of all forms and it really comes out in their music. Check some of them out if you haven't already.