Tests of waterproof materials generally measure two criteria:

    1. Rain-room endurance: rain-room testing is conducted primarily to meet International Customs requirements so fabrics can be classified as waterproof or not and receive the correct import duty. Rain-room testing is not generally a good measure of a fabric’s capabilities.
    
    2. Static column water resistance: designed to measure a material’s absolute water resistance, using a standard millimeter (or pounds-per-square-inch conversion) measurement. In static-column tests, a 1-inch diameter tube stands vertically over a piece of waterproof material; the tube is filled with water, and the measurement is the height of the water column in millimeters when leakage begins.

•0mm: obviously not waterproof in any way. Sieves, screen doors, basketball nets.

•0mm-1000mm: rain resistant, but not rainproof. Most stretch-woven softshells fall here.

•1000mm-5000mm: rainproof but not waterproof under pressure (sitting on or leaning against wet surfaces). Engineered (laminated) softshells, inexpensive rain shells, low-end ski and snowboard wear.

•5000mm-15,000mm: totally rainproof and generally waterproof unless under serious pressure (extended sitting, submersion, heavy people sitting). Most proprietary coatings (generally, liquid polyurethane coatings that become porous when applied to fabrics and cured) fall in this range.

•15,000mm-30,000mm: totally waterproof, even under serious pressure. High-end proprietary PU laminates, PTFE membranes like eVent and GORE-TEX® fabrics. Can withstand shallow-depth submersion without leaking (fishing waders, drysuits for sailing).

•35,000mm and up: Solid vessels and non-porous materials. Will deform or fail catastrophically before leaking. Nalgene bottles, rubber galoshes, aircraft carriers.

Remember: this is just the fabric. Not the garment itself. A garment constructed from 10,000mm-capable fabric is not necessarily 10,000mm waterproof. The fabric, often just the membrane/laminate on the fabric, is the only thing that was measured.
 

Information from http://backcountrybeacon.com/2010/04/waterproof-ratings-demystified/