credits
Credits are things that you can use to buy things in block miner.They are rewarded every 12 hours if you log in.The amount if credits you get varies but is usually 9-14 credits(not tested) with numbers outside this range occuring occasonally.
Coins
Coins can be used instead of credits to purchase items in block miner.They can be bought using Paypal in the "get coins"tab at the top of the screen.One coin is the equivalnt of 12 credits when buying an item.