Occasionally, in the discussions in the NodeJS mailing lists and IRC channels, you may hear things referred to as "node-core" and "userland".
Of course, traditionally, "userland" or "userspace" refer to everything outside the operating system kernel. In that sense, Node itself is a "userland" program.
However, in the context of NodeJS, "core" refers to the modules and bindings that are compiled into NodeJS. In general, they provide a hook into very well-understood low-level functionality which almost all networking programs are going to require: TCP, HTTP, DNS, the File System, child processes, and a few other things. If something is fancy enough to argue about, there's a good chance it won't be part of node-core. HTTP is about as big as it gets, and if it wasn't so popular, it'd certainly not be a part of node.
Everything else is "userland". This includes: npm, express, request, coffee-script, mysql clients, redis clients, and so on. You can often install these programs using npm.
The question of what is properly "node-core" and what belongs in "userland" is a constant battleground. In general, node is based on the philosophy that it should not come with "batteries included". It is easier to move things out of node-core than it is to move them in, which means that core modules must continually "pay rent" in terms of providing necessary functionality that nearly everyone finds valuable.
One goal of node's minimal core library is to encourage people to implement things in creative ways, without forcing their ideas onto everyone. With a tiny core and a vibrant user space, we can all flourish and experiment without the onerous burden of having to always agree all the time.
If anything, it's more. Building functionality in userland rather than in node-core means:
If you believe that something really just needs to be part of node's core library set, you should still build it as a module! It's much more likely to be pulled into node-core if people have a chance to see your great ideas in action, and if its core principles are iterated and polished and tested with real-world use.
Changing functionality that is included in node-core is very costly. We do it sometimes, but it's not easy, and carries a high risk of regressions. Better to experiment outside, and then pull it into node-core once it's stable. Once it's usable as a userland package, you may even find that it's less essential to node-core than you first thought.