Threadless used predominately basic screen printing techniques until 2006 when they launched a Select line with expanded inks and printing methods.
Now those inks and printing methods are open to all artists that allow for 8 colors, super glow, puff, high density, suede, UV color change, shimmer, metallic clear, glitter, blister, flock, vinyl, foils, embroidery, gradients, simulated process and belt printing.
You can check out the printing methods that Threadless uses below:
You can read more over on the Threadless Blog or go shop Threadless.