The public key is the location where transactions are deposited to and withdrawn from. This is also the key that appears on the blockchain ledger as a user’s digital signature, not unlike a username on a social media newsfeed. The private key is the password required to buy, sell, and trade the bitcoin in a wallet. A private key should be a guarded secret and only used to authorize bitcoin transmissions. Some users protect their private keys by encrypting a wallet with a strong password and, in some cases, by choosing the cold storage option; that is, storing the wallet offline.
Depending upon the exchange, there may be benefits and disadvantages to paying with cash, credit or debit card, or bank account transfer. For instance, while credit and debit cards are among the most user-friendly methods of payment, they tend to require identification and may also impose higher fees than other methods. Bank transfers, on the other hand, typically have low fees, but they may take longer than other payment methods.
Once you have a bitcoin wallet, you can use a traditional payment method such as a credit card, bank transfer (ACH), or debit card to buy bitcoins on a bitcoin exchange. The bitcoins are then transferred to your wallet. The availability of the above payment methods is subject to the area of jurisdiction and exchange chosen. Below is a screenshot of the bitcoin interface on Coinbase showing how to buy and sell bitcoin and also Bitcoin Cash, Ethereum, and Litecoin​, which are other popular virtual currencies. The user clicks the "Buy" tab to buy digital currency and the "Sell" tab to sell digital currency. You select which currency you are buying/selling and which payment method (your bank account or credit card) you want to use.