.
With end-to-end encryption (e2e), encryption and decryption always occur on the source and destination devices (such as on mobile phones for instant messaging). Encryption keys are generated on the client, so as an administrator, you don't have control over the keys on the clients and who can use them. In addition, you don't have visibility into which content users have encrypted.
With client-side encryption (CSE), encryption and decryption also always occur on the source and destination devices, which in this case are the clients' browsers. However, with CSE, clients use encryption keys that are generated and stored in a cloud-based key management service, so you can control the keys and who has access to them. For example, you can revoke a user's access to keys, even if that user generated them. Also, with CSE, you can monitor users' encrypted files.
A slow pup is a lazy dog. -- Willard Espy, "An Almanac of Words at Play"