BNT is the symbol for the Bancor Network Token. BNT is the world's first Smart Token™. BNT is the hub token that connects all tokens in the Bancor Network, allowing them to be easily convertible to each other.
BNT can always be purchased simply by sending ETH to an address — which instantly issues BNT tokens back to the sender, according to its current price, per the formulas in the Bancor Protocol white paper. Liquidating BNT to ETH is always possible as well through the same method. The total circulating supply of BNT is ever changing, BNT tokens are created when ETH is sent to their smart contract and destroyed when a user 'sells' their BNT back to the contract.