Screen Printing. 

"Knives" Andy Warhol

Screen printing is a pretty modern art form.  I'd venture to say that everyone in the united states has at some point owned or will own a piece of screen printed art.  T-shirts are the most common form, most people are actually unaware of the amount of products that are actually screen printed.  From coffee mugs to Frisbees, they screen it all.  

Pop Art:

With the emergence of pop art in the late 50's came a flood of artists working on a mass scale, screen printing was the obvious medium.  Andy Warhol being God. 
  

Today many artists are still keeping the tradition alive.  There are tens of thousands of t-shirt screen printers, ranging from commercial printing to small scale ma and pop shops printing local artists and business logos. 

"Meat" Roy Lichtenstein

There are multiple methods to make a screen print, but the basic concept is placing a resist on a fine screen which acts as the negative to the print.  The screen is then placed over the object being printed and ink is pressed through the screen leaving a print on the surface.  With this process it is easy to make multiple layers in an assembly line styled operation.  Ink is "set" to the object by applying heat, for cloth items a quick spin in the dryer is enough to make it permanent.