Online Trading Card Games (more commonly referred to as TCGs) are actually quite similar to the trading card games you knew as a child. The main difference is that it lacks all face-to-face interaction, and instead, takes place online through websites & emails. Instead of collecting cards from packs that you've purchased at a store, you'll play free games, which will earn you random digital cards that are yours to keep or trade away. The cards you receive are digital and kept safe through some form of self-hosted website or blog (such as my trading post that you've stumbled across), where you'll keep your cards organized and readily available for other members to view and offer you trades for.
The main goal is to collect all of the cards in a single deck, which is often referred to as "mastering" the deck. Throughout your journey, you'll be keeping the cards you like and/or need while trading the rest away to your fellow members.
TCGs are free to play, a lot of fun, and very addictive! Based off of past experiences, I can also confirm that they are incredibly competitive. (Trust me, I've only been playing on and off since Calico's original SCM. Over 15 years now...)
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