When you're discussing Topps vs OPC, there's a difference in hockey and baseball cards. Topps is the more desirable card for baseball, and OPC for hockey when they have both versions available. Has to do with print runs back when they were the only the only 2 kinds of cards.
As for some other stuff that's been mentioned about garded cards being the only ones of value ... yes that very true about the older cards (since anything of decent grade is rare as f*** - kids actually used to play with cards!) And there were sooooo many produced that even a 7/10 card probably still has thousands of copies out there.
For grading itself, there's really only 2 companies that the industry takes seriously... Beckett and PSA. PSA grades on 1 -10 scale using whole numbers on the overall card, whereas Beckett grades 1-10 using a .5 scale on 4 different aspects of the card - corners, edges, centering and the card face.
So while you might see a PSA card graded as a 9, that same card might be graded by Becket as an overall 9.5, - with subgrades listed as 9.5 corners, 9 edges, 9.5 face, 9.5 centering. Their system is much more detailed. Both companies will also authenticate signatures.
Other companies like KSA and GMA aren't really taken too seriously by collectors. Theres a couple other companies like ISA and SGC, but again, they're so small compared to the others that even seeing cards graded hy them is rare, with the exception of older cards. Nearly all new cards people have graded are sent to Beckett and PSA.
Thing is, cards these days are all being kept in such good shape by buyers and the cardboard stock is so much better quality than it used to be that even 9.5 is a relatively common grade. The 10 grade from Beckett is really really hard to get so any card rated 10 on all 4 aspects is given a special Black Label that sets it apart from even the other cards that are an overall 10, but only have 3/4 10s and 1 9.5.
A black label 10 beckett card will sell for many many times the book value of a card - book value generally being what a card rated as a 9 would be. That being said, book value is still highly variable depending on sport, geography, and the player.