The standard used to encrypt and digitally sign email messages with public key certificates is S/MIME.
S/MIME means Secure/Multipurpose Internet Mail Extensions. It extends the MIME format used for email attachments and rich content by adding cryptographic protection. With certificates issued for email identities, S/MIME can digitally sign a message to authenticate its sender and detect changes, and encrypt content so that only intended recipients can read it.
The alternatives describe related but different technologies. TLS protects the connection between mail software and a mail server, while SSL is its older predecessor; neither normally provides an enduring signature or message encryption after delivery. PGP, commonly encountered through OpenPGP implementations, can also encrypt and sign email, but it is not the certificate-based standard named in this question.
S/MIME support has appeared in major mail clients, including Microsoft Outlook, Apple Mail, Thunderbird, and GNOME Evolution. In practice, the sender and recipient generally need compatible certificates and appropriate trust chains, which is why ordinary webmail often relies on transport encryption instead.