Four basic things:
Pitch - distance between rivets;
Gauge - thickness of the drive links;
Drive Link Count - length of the loop;
Type - cutter shape and special features, eg: full chisel / semi-chisel cutters?, low-kickback?, narrow kerf?, skip tooth / full comp pattern?, etc.
The first three (mostly) determine if a loop will fit your saw/bar/sprocket combination, regardless of brand. The fourth determines it's intended use. Each manufacturer uses their own numbering system for marking their chains, and their own terms for common features (e.g. 'low profile' = 'Picco' = . . . ). Don't get hung up on that. Details for each manufacturer's chains are available on their respective websites:
Oregon: OREGON Maintenance and Safety Manual
STIHL: Chain Saw Chains - Oilomatic Saw Chains | STIHL USA Mobile
Carlton: 2013-2014 Carlton Catalog
Tri-Link: TriLink Products
Bailey's also has a good "Chain 101" section in their printed catalog (no longer produced), but you can ask for a PDF version here:
Husqvarna Chainsaws, Outdoor Power Equipment and Tree Care Supplies from Bailey's
Philbert
(P.S. - send me any you don't want - I'll find a home for them!)
Thanks for all the chain info. Found what I was looking for and much moore. There is a lot info on these manufacturer's web sites.