TLS (Transport Layer Security) is the protocol that provides encryption, integrity, and authentication between two parties; it is the successor to the older SSL (Secure Sockets Layer).
During the handshake it authenticates identities and securely agrees on session keys using asymmetric cryptography, then protects the actual data with fast symmetric encryption.
SSL 2.0/3.0 were deprecated due to serious vulnerabilities, so what people still call 'SSL' today is essentially always TLS. TLS 1.2 was widely used for years; TLS 1.3 cut the handshake to a single round trip and removed weak legacy cipher suites, improving both security and speed.
The core idea is hybrid: asymmetric keys (public/private) prove the server's identity and protect the session-key agreement, after which bulk data uses symmetric encryption for performance. Certificates are signed by CAs to form a chain of trust, and SNI lets one IP serve many domains. HTTPS, SMTPS, and QUIC all build on TLS (or its concepts) — QUIC integrates TLS 1.3 directly into the transport layer.