TCG is the usual acronym, "Trading card game". But rather than physical cards, it's little graphics made to look like cards, and rather than the focus being on some sort of game played with the cards, the collecting is the primary focus. The general... gameplay loop? Is that you play games to get cards, toss them on your trade post, trade them around with other members to get a full set of the same deck, and then turn those cards in for a badge saying you "mastered" the deck. It sort of requires having a place to display your cards (a "trade post"), and while you can put it anywhere you can freely display large amount of images (often Dreamwidth posts), it's a lot less tedious if you can use server-side scripts to do a lot of the legwork for you. As a result, there's always a handful of people in the community offering free hosting to other TCG players, and a lot of people touching FTP clients or PHP for the first time. (Though someone actually made a javascript-only script setup in recent years for one of the longer running TCGs so people could host their trade posts on Neocities!)

I've pretty much only ever played ones with some sort of fandom theme-- the only one I'm a member of currently is Sakura-- but I've seen ones around with other themes too, like desserts or locations.

Thank you!! I'll keep that in mind if I end up struggling once I've gotten my basic layout pinned down.