APR stands for Annual Percentage Rate. This is the percent that you will receive when staking with a validator. APR should not be treated as constant as it is influenced by a number of factors: the number of nodes, service fee, top-up amount of the pool and of the network, uptime rank. Remember that the more users start staking, the less APR in the network. You may find an actual APR in the explorers mentioned above.
APY stands for Annual Percentage Yield and is the projected rate of annual return after taking compounding interest into account. In order to understand how APY is calculated, it is important to understand compound interest. In essence, compound interest is interest earned on previously earned interest.