formatiing forms layout, grouped fields, headings, etc...

I've been doing a lot of work with different kinds of form fields in my custom content module...

what I'm wondering is if there's a way to add other elements to the form page, to create different sections, group fields together, maybe put some of them inside a div block with a border so the user can see them grouped together, and have a some h1 headings to describe the groups

also if you know where to find information on customizing form themes please feel free to post it here, I haven't found it yet....

thanks in advance!
EK