Blood and bones
A blood cost is a sacrifice count. The 2-blood Wolf needs two of your creatures to die before it hits the board, and Squirrels exist to be that fodder. Bones work the other way round: every creature of yours that dies adds one to the pile, so bone cards like the 4-bone Coyote get easier to afford the longer a fight drags on.
Energy and Mox
Act III runs on energy, and 68 of its 70 cards cost it. Your cap climbs by one each turn up to six and refills every turn. Mox belongs to Magnificus in Act II. A Mox card asks for a green, orange or blue gem already on your board rather than a payment, so losing the gem strands every card that needed its colour.
Act II mixes everything
Act II is the only act where all four costs show up together. Its packs hold 23 blood cards, 21 bone cards, 25 energy cards and 18 Mox cards, which is why so many Act II decks end up running two currencies at once.