Difference between 'Save' and 'Apply Changes'

When on many screens in your system, you have the choice to Save or Apply.

The difference is quite simple:

Apply means the changes will be applied and you will remain within the area where you are.

Save means the changes will be applied (saved) and you will exit to the main area you are in.

For example, if you are currently editing a customer, choosing Apply will save the changes on the page you are on and keep you within that specific customer. If you choose save, your changes will be saved and you will exit to the customer list.