Depends on the Powerbook model.
Ill cover all of the ones on market now.
12 inch Powerbook: Has a mini-DVI connector. Comes with an adaptor in box to use with a VGA monitor and another adaptor to use DVI. Max resultion on second display: 1600x1200 analog, 2048x 1536 digital.
15 inch powerbook: Full sized DVI port built in. Comes with a DVI-VGA adaptor. Max resultion on second display: 1600x1200 analog, 2048x 1536 digital.
17 inch Powerbook: Same as 15 inch.
All support both mirroring (same image on second display. Image only capable of being mirrored at the resolution of the primary display) and spanning (extends desktop space over to second display).