Both inline forms and pop-up up forms are types of lead-generating forms. However, they differ in the way it is being used.
While inline forms are embedded on a website page and would look like a part of it, pop-up forms would popup when a user is browsing the website. It is based on the preset conditions, allowing you to capture leads based on their behavioral patterns.
One instance would be when a user is about to leave the website, an exit popup form can be shown. Or, you can pop up a form when a user browses a web page for a certain amount of time.