The protocol commonly used for secure online transactions on the internet is Secure Sockets Layer (SSL), along with its successor, Transport Layer Security (TLS). These protocols establish an encrypted link between web servers and browsers, ensuring that sensitive data such as credit card numbers and personal information remain protected during transmission. SSL/TLS certificates are essential for websites that handle sensitive information, as they provide assurance to users about the legitimacy and security of a website. Using these protocols is critical for maintaining user trust in online transactions.