Most fabric manufactures have their own system to ID the various fabrics, though some commonalities exist, such as with "combo" fabrics like 1708. This is a 17 ounce fabric that's lightly bonded to 8 ounce mat. 1800 suggests it a straight 18 ounce fabric. There are different types of fabrics too, just to add to the confusion, such as weave type, fiber orientation, etc. Generally, these fabrics fall into a few different categories: mat, woven cloth (like the material in your tee shirt), directional fabrics and combo fabrics (more than one fabric in the same roll). These get broken down into specifics, such as weight, fiber orientation, etc.
http://www.diy-fiberglass-boat-repair.com/
http://www.shopmaninc.com/fiberglass.html
Download the free "user's guides" from westsystem.com, the "epoxy book" from system three.com and the "boatbuilding handbook" from westsystem.com for a more comprehensive listing.