How does Node.js perform compared to Apache?
Node.js is a framework particularly well suited for writing high performance web applications without having to understand how to implement concurrency at a low level. It is a framework for writing server-side JavaScript apps using non-blocking IO: passing continuations to IO calls rather than waiting on results. Node.js provides a system API (filesystem access, network access, etc.) where all of the API calls take a continuation which the runtime will execute later with the result, rather than block and return the result to the original caller.
You can use by itself, if you like. But you might want a dedicated reverse proxy in front of Node.js: something like Apache, Nginx, LigHTTPD, etc. Or, for clustering a bigger app, you might want something like HAProxy in front of multiple running Node.js app servers.
There is a recent (July 28th, published 30th) Google Tech Talk about Node.js where there are a few performance numbers and where he also talks about scaling.