In Node.js everything runs in parallel, except your code !!

In Node.js everything runs in parallel, except your code. What this means is that all I/O code that you write in Node.js is non-blocking, while (conversely) all non-I/O code that you write in Node.js is blocking.

http://greenash.net.au/thoughts/2012/11/nodejs-itself-is-blocking-only-its-io-is-non-blocking/