As Lars said, it depends. If it's a CLI tool or something similar, it's usually installed globally (npm install -g). Your project's dependencies such as Express, passport, body-parser, ejs etc are installed locally (npm install).
Okay. So, I have some packages (babel-cli, babelify, etc.) installed globally. Can I avoid re-downloading those when I need to use them for a project? Do I just "require" them?