a basic question from a total jquery/ajax newbie.

I have a simple form with two input fields for which I want the data passed via jquery and ajax to a php file on my server side. After processing, it should send me a nice reply back like "request was processed" or some other text. 

I have searched the web and this forum and I found that not all posts/webpages/tutorials describe this in the same way. So now, I am a bit lost in the abundance of information.

First question: in all tutorials, the form is inside an .html file (like index.html). In my case, the form is in a .php file. Does this have any influence in how the form is treated. The form is written in html syntax so it should not matter if it is an .html or in a .php file, correct?

Second question: 

In some tutorials, the form action will link to the php file where the data is sent. Method is usually "POST". 
Then, in the jquery file, the $.ajax url also points to that same php file.

Is this the correct way to do it?

If I set it up like that, upon clicking the submit button, the browser is always directed to the php file in the form-action and the jquery file I wrote is not executed. 
And I have no clue why this happens.

Hope someone can point me in the right direction.