What happen when npm install -g was executed

this is a very newbie question. But I really cannot figure it out through Google search. I want to understand what happens when one executes 

   sudo npm install -g "node module name"

I understand that the right module will be installed to, normally, /usr/local/.... However, why would this be available globally?

For example, I installed node-inspector locally in my project. But my shell does not understand when I type "node-inspector".

But once I install it globally,

node-inspector &

command will does the right thing for me. I really want to understand how this happens.

Thanks

Quoting the npm's doc:

  • Local install (default): puts stuff in ./node_modules of the current package root.
  • Global install (with -g): puts stuff in /usr/local or wherever node is installed.
  • Install it locally if you're going to require() it.
  • Install it globally if you're going to run it on the command line.
  • If you need both, then install it in both places, or use npm link.

It's quite a short description (which, I suppose, is still enough to see the difference), but the linked page describes the whole process of installing modules with npm in more details. )