What is a difference between installing npm module locally and globally?

When you install a module locally: 

• It will be installed into your local folder.
• The module is then not visible outside of this folder.
• You can have multiple installations of the same module in different folders.
• You can install different versions of the same module side by side.

When you install a module globally:

• It can be used from all the other directories.
• Only one version of the module can be installed.
• You should install a module globally when you want to use it in multiple different projects without having a maintenance overhead.