Linux – What happens when npm install -g is executed

This is a very newbie question. But I really can’t figure it out by googling.
I want to understand what happens when executed

   sudo npm install -g "node module name"

I know the correct module is usually installed to /usr/local/….
But why is this available globally?

For example, I installed node-inspector locally in the project. But when I type “node-inspector”, my shell doesn’t understand.

But once I installed it globally,

node-inspector &

The command will do the right thing for me.
I’m really interested to understand how this happened.

Thanks

Quote 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.

This is a fairly brief description (which, I think, is still enough to see the difference), but the link page describes in more detail the entire process of installing modules using npm. )