Macro packages
Macros build on groff's low-level faicilities and are used to specify how certain elements (such as starting a paragraph, layout of headers and footers etc.) should be executed.
The most common macro packages are man, mdoc, me, ms and mm (With man probably being the most popular).
The command line option -m specifies the macro package to be used for processing:
$ groff -m man …
Devices
Man pages can be created for different devices. When invoking groff, the device can be stated with the -T option. Some interesting devices, imho, are: dvi, Xn (n=75, 100 etc) for X11 previewer, ascii, latin1, utf8, html.