Forms allow you the flexibility to have just the form itself to be embedded in another web page, such as on your website, or to be used as a pop up lightbox style form on any website. You can design a form to fit into a site's sidebar, or pop up when they scroll 50% down the page, or pop up when they move their cursor up to the top of the page (called "exit intent").
Pages are usually used as stand-alone web pages that you can publish either as an independent page on the web, or as part of your WordPress website using our ONTRApages Plugin. A typical use is an opt in or sales page with an embedded form. You can publish this page independently on it's own domain, and drive traffic to it. Or, you can publish this page as a page on your WordPress website using the ONTRApages Plugin. But you couldn't publish the page in the side bar of your WordPress site easily, and a complete web page can't be a pop up without obscuring the entire screen.