nodejs/doc/api/cluster.markdown

Cluster

A single instance of Node runs in a single thread. To take advantage of multi-core systems the user will sometimes want to launch a cluster of Node processes to handle the load.

By starting node with the cluster argument, Node will detect the number of CPUs on the machine and start that many processes. For example suppose we had a simple HTTP server in server.js:

require('http').createServer(function(req, res) {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

If we start it like this

% node cluster server.js 
Detected 2 cpus
Worker 2438 online
Worker 2437 online

Node will automatically share port 8000 between the multiple instances.