Description from cryptocompare.com
The Bancor Protocol is a blockchain-based system for discovery and a liquidity mechanism supporting multiple smart contract platforms. The flexibility of these blockchains allows tokens to be locked in reserve and to issue smart tokens on the Bancor system, enabling anyone to instantly purchase or liquidate the smart token in exchange for any of its reserve tokens.
The BNT is the first smart token on the Bancor system and it will hold a single reserve in Ether. Other smart tokens, by using BNT as one of their reserves, connect to the BNT network. The BNT establishes network dynamics where increased demand for any of the network’s smart tokens increases demand for the common BNT, benefiting all other smart tokens holding it in reserve.