Node is a platform for writing JavaScript applications outside web browsers. This is not the JavaScript we are familiar with in web browsers! There is no DOM built into Node, nor any other browser capability, but a DOM can be added using JSDom, and there are Node-based wrappers available for some browser engines. Between the JavaScript language and its asynchronous I/O framework, it is a powerful application development platform.
Beyond its native ability to execute JavaScript, the bundled modules provide the following capabilities:
Command-line tools (in shell script style)
Interactive-TTY style of program (REPL or Read-Eval-Print Loop)
Excellent process control functions to oversee child processes
A buffer object to deal with binary data
TCP or UDP sockets with comprehensive event-driven callbacks
DNS lookup
Layered on top of the TCP library is an HTTP and HTTPS client/server
File system access
Built-in rudimentary unit testing support through assertions
The network layer of Node is low level while being simple to use. For example, the HTTP modules allow you to write an HTTP server (or client) in a few lines of code. This is powerful, but it puts you, the programmer, very close to the protocol requests and makes you implement precisely which HTTP headers to return in request responses. Where a PHP programmer generally doesn't care about the headers, a Node programmer does.
In other words, it's very easy to write an HTTP server in Node, but the typical web application developer doesn't need to work at that level of detail. For example, PHP coders assume Apache is already there, and that they don't have to implement the HTTP server portion of the stack. The Node community has developed a wide range of web application frameworks, such as Connect, allowing developers to quickly configure an HTTP server that provides all of the basics we've come to expect—sessions, cookies, serving static files, logging, and so on—thus letting developers focus on their business logic.
Quit scratching your head already. Of course you're doing it, scratching your head and mumbling to yourself, "What's a browser language doing on the server?" In truth, JavaScript has a long and largely unknown history outside the browser. JavaScript is a programming language, just like any other language, and the better question to ask is "Why should JavaScript remain trapped inside browsers?"
Back in the dawn of the web age, the tools for writing web applications were at a fledgling stage. Some were experimenting with Perl or TCL to write CGI scripts, the PHP and Java languages had just been developed, and even JavaScript was being used on the server side. One early web application server was Netscape's LiveWire server, which used JavaScript. Some versions of Microsoft's ASP used JScript, their version of JavaScript. A more recent server-side JavaScript project is the RingoJS application framework in the Java universe. It's built on top of Rhino, a JavaScript implementation written in Java. All this means that JavaScript outside the browser is not a new thing, even if it is uncommon.
Node brings to the table a combination never seen before; namely, the coupling of fast event-driven I/O and a fast modern JavaScript engine such as Google's V8, the ultrafast JavaScript engine at the heart of Google's Chrome web browser.