That really depends. If you build your application on Node, you don't need
Tomcat, but Node is certainly not a drop-in replacement for Tomcat.
Node and Tomcat are two very different things. Tomcat is a server for
Java applications with a lot of built-in features and extras that are available with no add-ons.
Node is a server-side JavaScript environment that powers JavaScript applications. It is not a server, but you can build servers in Node with a few lines of code.
Node features:
A fast JavaScript engine (built on V8).
Asynchronous by default
philosophy (nothing should block).
Event-loop design (much like the browser environment).
Networking as a first class citizen (create production capable servers with few lines of code).
A highly usable streams API.
A large, rapidly growing developer community.
A simple, CommonJS-based module solution that guarantees module encapsulation (your var declarations are limited to module scope).
A developer friendly package management system with thousands of open-source packages to choose
from.
To learn more about node, see my recent blog post:
Getting Started with Node and Express.