BNB is a cryptocurrency (token) that was created by Binance back in 2017. It was first issued and launched during an Initial Coin Offering (ICO) crowdfunding event, which took place between the 26th of June to the 3rd of July. 

The token sale offered 100,000,000 units of BNB (50% of the total supply). The issue price was 1 ETH for 2,700 BNB or 1 BTC for 20,000 BNB (around 0.11 USD back then).

BNB was initially issued as an ERC-20 token (on the Ethereum platform), but it has since been transferred to the main BNB Chain. The initial total supply of BNB was defined as 200,000,000 coins, but due to the regular coin burn events, the supply is gradually decreasing. To learn more about BNB coin burns, please refer to this video at Binance Academy.
BNB has multiple use cases, but within the Binance Exchange ecosystem, it is used as a utility token that allows users to receive discounts when paying for their trading fees. You can check the discount schedule in this article. Moreover, the fee structure also changes according to the trading volume (30 days) and tier level of the account (more details on this page).
Besides being used by traders to get discounts on their trading fees, BNBs can also be used to pay for travel expenses (such as hotels and flights booking), to buy virtual gifts, to shop using a credit card, and much more
In addition, you can also make impactful donations to charity through the Binance Charity Foundation (BCF) project. BNB can also be used as gas on the BNB Chain. If you are not sure how to buy BNB directly, go to the Binance exchange.