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SSH, short for Secure Shell, is a cryptographic network protocol that helps to run network services securely across an insecure network, like the internet.

It basically offers an encrypted and authenticated method for accessing and controlling remote systems (such as servers) and transferring files securely.
Key Functions and Features
Secure Remote Login and Command Execution:
With SSH, one can remotely log into a computer and run commands as if physically at the machine, with the entire communications session encrypted. This is its most typical use.
Encryption:
Everything, from login credentials (username/password or private key) to the commands/output, is encrypted. This keeps the session secure from eavesdropping, hijacking, and other network-based attacks.
Authentication:
SSH employs cryptography (usually public-key pairs) to authenticate both the client (user or machine) and the server, that they are who they say they are.
Secure File Transfer:
SSH offers secure file transfer features through associated protocols such as SFTP (SSH File Transfer Protocol) and SCP (Secure Copy Protocol).
Tunneling/Port Forwarding:
It can securely "tunnel" or forward other network connections over the encrypted SSH channel, allowing access to services that might otherwise be blocked or insecure.
How It Works (Client-Server Model)
SSH operates using a client-server architecture:
Client Initiates:
The SSH client (the program on your local machine) initiates a connection to the SSH server (the program running on the remote machine, often called sshd).
Negotiation:
The server and client negotiate encryption methods and key exchange to create a secure transport layer. They create a shared session key for symmetric encryption, which is quicker and with which they encrypt all subsequent data.
Server Authentication:
The client checks the identity of the server, typically by comparing its Host Key to a pre-known list in order to avoid Man-in-the-Middle attacks.
User Authentication:
The user is authenticated by the server. This is achieved in one of several methods:
Password Authentication: The user enters a username and password that are sent securely.
Public Key Authentication: This is the most popular and safest method. The user possesses a public/private key pair. The server keeps the user's public key, and the client uses the private key to authenticate itself without sending a password.
Secure Session:
After authentication, a secure, encrypted tunnel (the "shell") is set up, through which the user can run commands on the remote host.
SSH was designed as a replacement for older, insecure protocols such as Telnet and FTP, which sent data unencrypted, and thus were open to interception. It generally listens on TCP port 22 by default.