A page presents information to a user. It can also collect information entered by a user. A page is displayed in a browser.
A form is a page that belongs to a process. In a process, there can be forms for process instantiation and for human tasks, and there can be overview forms.
You are recommended to use the UI designer to create forms for Bonita BPM 7.x processes. UI designer forms enable you to take full advantage of the separation between the process definition and the UI definition, by using contracts.