What do network protocols define and why should secure versions be used?

Protocols define the message types, syntax, field meanings, send/response behavior, and lower-layer interaction of a communication session; secure versions (SSH, SFTP, HTTPS) should be used whenever possible.

A protocol is essentially an agreed contract for a conversation between two machines: because the two ends are often different vendors and operating systems, they can only interoperate if every detail is pinned down in advance. That's what each item in the list does — it fixes the processes at each end, the types of messages allowed, their syntax (exact format), the meaning of each field, the send-and-response choreography (what reply a message expects), and how the protocol hands data to the layer below. Get any of these wrong and the other side can't parse you. The push toward secure versions (SSH for Telnet, SFTP for FTP, HTTPS for HTTP) is a separate but related point: the original protocols carry credentials and data in plaintext, so anyone capturing the traffic can read it — the secure variants wrap the same contract in encryption.

Network Protocols:

Network protocols support the applications and services used by employees in a small network.

Protocols define:

• Processes on either end of a communication session
• Types of messages
• Syntax of the messages
• Meaning of informational fields
• How messages are sent and the expected response
• Interaction with the next lower layer

Common Protocols:

Security Policy: Many companies have established a policy of using secure versions (SSH, SFTP, HTTPS) of these protocols whenever possible.

Go deeper:

• Wikipedia — Secure Shell (SSH) — how the encrypted replacement for Telnet hides credentials a plaintext protocol would expose.