You could probably put together a collection of 100,000 decks of modern playing cards without duplication. Yes there are differences.
Firstly, cards can be made of paper coated in plastic ("paper cards") or 100% plastic ("plastic cards"). Plastic cards are recommended for poker because they shuffle more easily, last longer (a lot longer), and they slide more easily for dealing.
Size - "poker" size (fatter) and"blackjack" size (norrower) are the most common. Both work fine.
When people say "shirt" I assume they are referring to the pattern on the backs of the cards. In addition to pattern, back designs differ in style as well. The most common are "full bleed" where the pattern runs to the edge of the deck and bordered (? - don't know the name) cards with a border surrounding the pattern. Either one is fine for poker.
The face of the cards presents the most options. In addition to choosing a design for the pips and faces, as well as any background graphic variations, you have options of standard or four colour deck, and index design.
Index Design
This is the variant that is most important to most players. Index can be standard size or large, “peek” – present on both sides of each card, or on the middle of the long side (don’t know what this is called, most people hate it).
I recommend that you go to a site that sells cards and look at the huge variety available. The most important factor to determine by “feel” is the difference between paper and plastic cards. Once you have done that, the remaining choices can be made relatively confidently by sight.
