Best thing to do is find your local paper suppliers and talk with them about their services. Partnering with a local distributor can save you hours and headaches. Kelly Paper, Xpedex, etc are great sources of information and help if you're confused about paper.
Each distributor usually has a couple of paper manufacturers that they distribute for and each paper manufacturer will have variations on the standard papers. We use Xpedex in my current, mid-sized shop who deal mostly with International Paper (the new-ish 50/10 Mohawk series is pretty nice). My previous paper supplier was Kelly Paper who have a ton of Wausau offerings. My screenprinting operation used almost exclusively Mr. French paper (very heavy stock). The supply chain usually goes: Paper Mill to Paper Supplier to Printer to End User. It's important to be sure your supplier has what you need for your end user.
For most business to customer type printing (copy shop, quick print, small shop) you'll want to be sure to have a cover and text weight standard paper in both coated and uncoated in tabloid (11"x17") and standard letter (8.5"x11"). Other papers that are very common if not standard are gloss coated 1-side ("C1S"), gloss coated 2-sides ("C2S"); a nice smooth uncoated cover (I prefer Classic Crest); and the ubiquitous 24-28 pound ("24-28#") "copy"/"bond"/"laser" paper.
The specific paper weights (or micrometer "points") that you choose is really just a function of your output device (copier, press, etc) and your clients needs. Check with your press crew and press/copier user manual for acceptable paper weight ranges. For instance, my konica 6500 won't take super-heavy cover papers but my presses easily do.
Larger shops will usually trim down to their needs from parent sheets but if you're ordering large parent sheets you should seriously talk to your potential paper merchants before making any purchases.
Which paper mill product lines you choose really depends on your budget, your clients needs and your printing process. Most mills now offer a standard group of papers for the most common applications. One thing to be sure to ask from a potential paper supplier is a sample book. That will clear many of the more basic questions right away and help familiarize you with paper naming conventions.
Sorry for the wall of text, hope some of my ramblings help. G'luck!
