End-to-End Encryption is a general name for every system of communication where the entire encryption and decryption processes are taking place on the client-side only.
Theoretically, that means that only the communicating users can access the delivered data- the server, or any third-party service/person, won't be able to decipher the data.

For example, Gmail's HTTPS connection (SSL/TLS protocols) is not an end to end encryption - the traffic is fully encrypted between the client and the server, but the entire data is being kept deciphered on the company's servers.