BNB is an ERC20 token built on Ethereum. The total supply is limited to 200 million BNB, after which no more coins will be created.
You can use BNB to pay your fees on the Binance exchange. While you can still choose to pay your fees in the cryptocurrency you’re exchanging, Binance offers a rebate as an incentive to pay fees in BNB. In your first year of membership on Binance, fees are discounted 50% when paid in BNB. With every subsequent year of membership, the fee discount decreases. By your fifth year using Binance, there is no longer a fee discount for paying in BNB.