Definition
End-to-end encryption is the strongest form of encryption-in-transit because the keys exist only on the two endpoints. A messenger, a server, an ISP, or a government surveilling the wire all see only opaque ciphertext. Signal Protocol, used by Signal, WhatsApp, and others, is the canonical modern example. ProtonMail is an E2EE email provider; iMessage between Apple users is E2EE; banking apps frequently use E2EE for sensitive fields.
Why It Is Not What a VPN Does
A common misconception is that a VPN provides end-to-end encryption. It does not. A VPN encrypts traffic between your device and the VPN server. After the server, traffic continues onto the public internet in whatever form the destination expects — encrypted (HTTPS) or not. The VPN server itself decrypts and re-routes everything, which means it could in principle read or log unencrypted destinations. E2EE is the application-layer guarantee; VPN is the network-layer guarantee. The two complement each other rather than overlapping.
Layered Protection
The strongest practical setup combines both: HTTPS or another E2EE protocol protects the content from the destination back to your device, and a VPN protects the metadata (which sites you visit, when, from where) from your local network and ISP. Defeating both simultaneously requires breaking the application-layer cryptography and either compromising your VPN provider or controlling the network on both sides of the tunnel.