what is the difference between
npm install express
npm install -g express
if you can also point me to a link which explains these options it will be very useful
See https://npmjs.org/doc/npm.html
npm has two modes of operation:
global mode: npm installs packages into the install prefix at prefix/lib/node_modules and bins are installed in prefix/bin.
local mode: npm installs packages into the current project directory, which defaults to the current working directory. Packages are installed to ./node_modules, and bins are installed to ./node_modules/.bin. Local mode is the default.
Use --global or -g on any command to operate in global mode instead.
The Google search phrase I used was:
npm options "-g"