In a nutshell, a secured credit card requires a cash deposit in order to reduce risks to the credit card company. This is a great option for individuals trying to repair poor credit, or for those still building their credit. The amount of your deposit is usually also the amount of your credit limit (i.e. if you deposit $250 your spending limit will be $250). In the event you don’t pay your bill, the credit card company will simply take the money from your deposit. But if you pay responsibly and build a positive track record, you could quickly qualify for an unsecured card (one that does not require a deposit).
You can use a secured card wherever credit cards are accepted, including online, but remember that if you carry a balance interest charges will apply. Some secured cards come with zero annual fees but most will charge an annual fee.