How does PayPal work?

Can you please explain in simple terms how PayPal works? I've heard it's a convenient way to send and receive money online, but I'm not quite sure how the process goes. For instance, do I need to have a PayPal account to use it? How do I add funds to my account? And how do I ensure that my transactions are secure and protected from fraud? I'm also curious about the fees associated with using PayPal, and if there are any limitations to the amount of money I can send or receive. Any insight you can provide would be greatly appreciated.