The ngSubmit directive binds to the submit event in the browser, which is fired when a form is submitted.
From MDN:
Note that submit is fired only on the form element, not the button or submit input. (Forms are submitted, not buttons.)
So you might use it to submit a user sign-up form, or something like that.
On the other hand, the ngClick directive can apply to any kind of element.
From source:
The ngClick directive allows you to specify custom behavior when an
element is clicked.
Use it to allow your user to interact with your page in some way other than submitting a form. Maybe to click on a ‘previous’ or ‘next’ pager button, or maybe a map or something.