Tomorrow Exchange is a distributed cryptocurrency exchange, a sweet spot between centralized and decentralized (non-existent at all yet) exchanges, combining security and usability of both.
Instead of going to the exchange website, you download a light weight Open Source client which runs on your computer. It has a multi-currency wallet functionality, as well as providing the way to trade the currencies safely. The exchange servers never have your private keys, and the money are either securely stored on your device and controlled by you (Satoshi’s way) or are in transit on the order, but with the signed guarantee from the exchange, making it cryptographically accountable for every satoshi it operates with. See the Whitepaper for the detailed description of inner workings.