Digital currencies are peer to peer networks. It is important to have many people participate in the network for network security. To ensure this, some kind of motivation is needed. Bay is a Proof of Stake (PoS) coin. That is geek talk for Bay paying interest to those that help strenghten the network.
Staking, as this is called, is to leave your wallet running, and thus become one of many nodes that confirm transactions on the network. The interest rate is 1% annual.
The QT wallet can receive, store, and send Bay. It can also stake.
The Client can send, receive and store Bay. It also holds the market and all the advanced functions of Bay. It can however not stake yet. That will come in a later update. For now you can only use the custom contract to sell on the market. Templates for the other categories are in the making, and will come soon with the next update.