From the FAQ:
Isn’t Bancor just a decentralized exchange?
Bancor is not an exchange. It is a protocol which allows for asynchronous price discovery and continuous liquidity. This means that converting a smart token does not require matching two parties in real time with opposite wants, rather, it can be completed by a single party directly through the token’s smart contract.
So, unlike exchanges where you have to find someone who actively wants to buy what you’re selling, smart tokens are always completely and immediately liquid regardless of trade volume. This enables a long tail of countless potential currencies that normally couldn’t achieve liquidity because their trading volume is too low or their use is too specialized. Smart tokens built using the Bancor protocol form an interconnected monetary network that instantly facilitates any request to convert between tokens in the network.