Do you remember that sweet sweet feeling of discovering how something works from your earliest gaming experiences? When all you could do was "f*** around, find out" since you basically couldn't read anyway? I loved it!
I wanted to make a game with very little hand-holding. The players would have to figure out why some cards are more powerful than others, figure out how the merging cards function is triggered and what it does and so on. But since the question has been asked over and over again here you go, a short explanation:
TIER: Cards have a tier, the tier determine the base power. Tier 1 is very strong, tier 150 is stupidly week.
ART: Card power is modified by card apperance. Full art cards are stronger. Foil cards are stronger.
MODS: Each hard have an individual modifier, so two cards of the same tier can have different base power.
TYPE: The cards have types, some types are strong against other types. Use it. Abuse it!
So now you know :)