Popup Forms are like Landing Pages, with one important difference:
Landing Page: When you are on a website and click a button or link to take you to a Landing Page, you leave the site you are on and you go to a different URL where you will find the Landing Page.
Popup Form: When you are on a website and click a Popup Form Button, a form will popup as a modal on the website you are on. In other words, it does NOT leave the site you are on and take you to a different URL.
Here is an example of what a Popup form could look like: