What hook to listen to a form submit?

[seedingbox]

I'm writing a module that acts on another module. The other module's submit form is at admin/settings/image-toolkit. When its form is submitted, my module needs to respond to that event. What hook do I need to listen for and how do I know the name of the form?I'm not even sure where to print dsm in this case to get more information about this form. Is there something like hook_nodeapi but for forms that I could give me more info about the form?