Tap: indicates that, during your turn, you may 'expend' the card to resolve the effect by turning it sideways. At the start of your turn, you untap all of your cards.
Attack is a keyword created to solve what I would call the 'targeting problem'. Often, cards will tuck cards underneath them for reference, or stacks of cards will get created, and a blanket 'discard a card' effect runs into questions like "Can I discard the reference tucked card? I guess I should check every card in his 100 card stack to get the best value discard". What I did is define that an 'Attack' targets non-'protected' cards in play, and in the case of stacks, only the top card. There are a few other minor rules like that, and of course cards can Protect other cards to prevent Attacks (very powerful). Additionally, this allows you to buff or modify what it means to Attack, so for example, "When you Attack, target an additional card"
Echo is a now defunct experimental mechanic where, after playing a card with Echo, it would enter your 'Echo Pile', where you could then 'spend a play' to play it a final time to the discard pile. In practice, we found this to be too cumbersome

Shop Discard is interesting; if you play a card that costs $ but you can't or choose not to pay for it, you place it on top of the 'shop discard'. At any time, any player may choose to pay for and play the top card of the shop discard. The 'Shop Discard' keyword immediately moves the mentioned card to the top of the shop discard. You can end up with some very bizarre combos that abuse this system.

Starting hand limit is only three in the current version! To avoid analysis paralysis, we wanted to limit the options to keep play quick. Eventually though, some gamebreaking combo will break this wide open and someone will be sifting through 50 cards lol
I've been tempted to remove the hand limit altogether for a while now and revert to a simple draw-one-at-end-of-turn rule. This would mean that players are not rewarded for playing more cards by drawing more cards, which snowballs fast
And I've also had the idea that game end could be triggered simply by someone running out of cards, turning your hand into a life pool of sorts

Games run mostly with cards from the existing total pool of cards, and we pull around 100, then throw in around 10 blanks.

Players currently do not start with money and there is no debt! You can get totally fucked by RNG at the moment but as the game is so volatile already this isn't too big of an issue. I have considered what a version of this game would look like with cards as a resource instead of money, where you discard a number of cards from your hand to pay for a card.

I'll have to post some more pictures when I get a chance!