First off, welcome to Call of Cthulhu. Now to answer your questions:
A great place to check everything out is CardGameDB. Like the name implies, it's a database of all the Cthulhu cards complete with scans and which packs or boxed sets they come in.
There are 3 types of products in the Cthulhu line.
1. The Core Set
This is the base game. It comes with x1 of every card included (total of 155 cards and 10 story cards), has success tokens, domain markers, and everything else you need to start playing.
2. Deluxe Expansions (aka Boxed Expansions and Boxed Sets)
These sets usually include x3 of every card (averaging a total of 165 cards), though there is one exception. Some are faction specific and some are more generalized. The main difference between these and the Core set is that they are usually more focused around a particular faction or theme, and don't come with the main rulebook or other accessories you need to play.
Last year FFG announced they would stop production of the smaller, monthly releases for CoC known as Asylum Packs and only put out larger boxed sets 3-4 times per year. This new release schedule started with Seekers of Knowledge, then was followed by The Key and the Gate, Terror in Venice and now the upcoming Denizens of the Underworld.
You'll see a pattern here. FFG started with a specific "human" faction, then a specific "monster" faction, then a themed set for all the factions, then back to a specific "human" faction set again. While it's only a guess, we expect them to continue this pattern. Therefore, after Denizens of the Underworld comes out (which focuses on the human Syndicate faction), we should see a monster-specific deluxe set.
Here's a specific breakdown of the various deluxe expansions in order of release:
A) Secrets of Arkham (SoA)
It's a generalized expansion, meaning you get cards from all the various factions except The Order of the Silver Twilight (as that faction hadn't been released yet). You also get a new set of 10 story cards (this is the only deluxe expansion to date that includes a story card deck).
Note, Secrets of Arkham comes with x2 of every card included, as opposed to x3 like all the rest. Therefore, to get 3 copies of every card you have to purchase 2 Secrets of Arkham sets. You will have half a set of cards leftover, but you can easily sell these online.
B) Order of the Silver Twilight (OotST)
This is the first deluxe expansion to focus on a single faction, as well as contain x3 copies of every card. It introduces the Silver Twilight to the game so the majority of the cards are for that faction (40 out of 55), though it includes 15 different cards for the other 7 factions (roughly 2 each).
C) Seekers of Knowledge (SoK)
This deluxe expansion focuses primarily on the Miskatonic University faction. It contains x3 copies of every card. It's considered a strong expansion and makes playing mono-M.U. a viable option.
D) The Key and the Gate (TKatG)
This deluxe expansion focuses on the Yog-Sothoth faction. Again, it contains x3 copies of every card. It's trickier to use than Seekers of Knolwedge, but does introduce a new mechanic where you trigger cards from your discard pile which can be quite powerful.
E) Terror in Venice (TiV)
Similar to Secrets of Arkham, this is a generalized expansion, containing a roughly even number of cards for each faction. It has x3 copies of every card. Terror in Venice has two primary themes - the day/night mechanic and conspiracy cards.
F) Denizens of the Underworld (DotU)
This hasn't been released yet. It focuses on the Syndicate expansion and will have x3 copies of each card.
And that's what we have so far for deluxe expansions.
This post is getting long so I'll wrap up for the moment and be back to tackle Asylum Packs later on.
Does this answer some of your questions? If not, please don't hesitate to ask. I'm happy to help.
On a final note, head over to the Call of Cthulhu forums at http://www.cardgamedb.com/forums/. They see more action than these forums and are a better place to chat (which is probably FFG acquired that website last month).
Edited by Yipe, 29 September 2013 - 12:24 PM.