Posted by Neil Sotirakopoulos on December 9th 2009

Short answer - Laminates and coatings are a key part of any fabric membrane responsible for keeping the water out while still letting your jacket breath. DWR, or Durable Water Repellent is a coating that prevents the face fabric from absorbing moisture. Seam tape reinforces all the stitches in the jacket and seals them so that they are totally waterproof, and scrim is an extremely lightweight quilted material that reinforces the insulation to make sure it stays together, in place, and evenly distributed every time you eat it on a handrail or send your jacket to the laundromat.

Long answer - Read these links on Wikipedia:
Waterproof Fabric
Membrane
Durable Water Repellent
Scrim
Thermal Insulation

Alternative Answer - Watch & enjoy this very non-scientific video: