What does npm intall actually do?

Lately I am diving into the nodejs world. I would said it's my bet on the future of programming, JS will rule the world (fingercrossed). So back to the topic of this post, while I was cloning different nodejs project from git, I notice everyone is using "npm install", and I am curious of what does "npm intall" actually do, first I ask some colleagues who worked closely with nodejs, but I cannot find a satisfying answer. Therefore, I asked the ultimate expert "Google", what I got is really informative and help me understand how "npm install" work.

In short (as said in the npm install doc):

By default, npm install will install all modules listed as dependencies inpackage.json

Yet, npm install could install from many other location / file other than the package.json in the root folder of your project.

A package is:

• a) a folder containing a program described by a package.json file
• b) a gzipped tarball containing (a)
• c) a url that resolves to (b)
• d) a <name>@<version> that is published on the registry (see npm-registry) with (c)
• e) a <name>@<tag> (see npm-dist-tag) that points to (d)
• f) a <name> that has a "latest" tag satisfying (e)
• g) a <git remote url> that resolves to (a)

You can read more by going to install | npm Documentation