Wandering the isles of coral at the LFS, I'm always amazed at the variations in color n structure. Short of asking every few seconds "Hey what is that called!", what is the best resource for looking up coral varieties?
I learned by asking the local fish stores and honestly, that will be the best way to ID corals. It sounds like you're interested in learning about the GROUPS of corals.
Some corals can be grouped based on features but if you're just starting out. It's best to go to stores and ask. Because just looking at pictures, its hard to learn the differences between some acans and favias. And there are tons of corals that look similar but are different.
The resources that I use for general coral group identification are (coral vendors):
I am relatively comfortable distinguishing the various family of coral. I'm more interested in the name of a particular piece. Eg all the variations of zoa or Acro or chalice. What is that color combo called? Or what type exactly is that green ringed blue centered zoa
Keep in mind, ordinary people are naming these corals (these are common names and are not scientific in anyway). And there is a high incentive for stores/hobbyists to give names to corals as a way to sell them for higher prices.
Often times when people "discover" a coral, they'll spend 20 minutes looking for it on these websites and then think they truley discovered something unnamed.
So you'll see corals with multiple names, which makes things confusing at times. Examples: Pink Hippo Zoas = Chuckie's Bride Zoa. I like the idea of coral names because it allows people to communicate quickly instead of saying that "zoa with the red skirt and red ring and blue center" (red hornet zoa). I don't like names for the sake of selling.
Same method my wife uses. Most of her names end in 'thingie'
There was an article in, IIRC, reef2reef, proposing a zoa naming convention. All those made up names are just crazy. How in the world can a zoa be named 'Gorilla nipples' and NOT be completely black.
I used to use coral pedia, but now you have the same type of coral there named several different names. just cause it has a slight difference in looks or another online vendor is tagging their name along with it. blah.
Plus there are some forums that cater to that whole NAMING high price mumbojumbo