Trading Card Sets
What is a trading card?
All anime trading cards are part of a larger category of "character goods." "Cards" means that they are flat. Size is not relevant. Some cards are tiny, some are huge. Nor, for our purposes, does it refer to material or purpose. Stickers, bookmarks, magnets, etc, are all cards to us.
"Trading" refers to the fact that you usually don't know exactly what you're getting, as opposed to a shitajiki or lami, and you may have to trade with others to get a full set.
If you still don't know after that, you just have to eyeball it. :)