This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold email@example.com with numerous contributions by others.
The Command-Shift-3 FKey that's built into all Macs will take
a picture of the entire screen. This won't work while a menu is
pulled down and always includes the cursor in the picture. In
System 6 Command-Shift-3 only works with black and white monitors
on compact Macs. The results are stored in a PICT file on the
root level of your System disk.
Nobu Toge's Flash-It, $15 shareware, will handle almost all
your screen capture needs. It works in black and white and color
under both System 6 and System 7, exports images to the clipboard
or to PICT files, captures pictures when menus are down, and can
capture either a user-selectable region or the entire screen. See
The Beale Street Group's Exposure Pro ($78 street) covers all the
basics and throws in a host of editing tools besides. Sabastian
Software offers Image Grabber ($35 street) whose features include
timed capture, capture of the entire screen, one window, or a
particular rectangle, and scaling of the captured image.
If you order Image Grabber, please note the spelling. It's two
words, spelled correctly. Apparently a grammatical product name is
so unusual that three out of three mail-order companies were unable
to find Image Grabber in their database until I spelled it out for
them including the space between Image and Grabber. You can also
order it directly from the manufacturer at (206) 865-9343.