In the context of node.js, a server is a program that listens for incoming network requests and responds to those requests by executing code or returning data. Servers are fundamental to the architecture of web applications and enable communication between clients (such as mobile apps or websites) and databases or other backend services.

Here is an example of a simple server written in node.js that listens on port 3000 and responds to all incoming requests with "Hello, World!":

const http = require('http');

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello, World!');
});

server.listen(3000, () => {
  console.log('Server running on port 3000');
});

One key difference between a server and a client app like a mobile app or website is that the server is responsible for managing data and state, while the client is responsible for presenting that data to the user. The server runs continuously and is always available to respond to requests, while the client is only active when the user is interacting with it.

Another key difference is that the server typically runs on a different machine than the client app. This allows the server to handle heavy computational tasks and manage data storage and retrieval, while the client app can focus on providing a smooth user experience.

Overall, servers are a critical component of modern web applications and provide the backbone for communication between clients and backend services.