I know what shift and control are, but what are the "command", "cmd", and "option" modifiers?
In addition to the usual Shift and Control keys, Macs have Command and Option keys.
For Squeak, the Command key on a Mac is treated the same as an Alt key on Windows or Unix. The Option key has no equivalent on most non-Macs, but its use is being phased out of Squeak.
Unfortunately, the default mapping of non-Mac Alt to Mac Command makes for unfamiliar keystroke shortcuts for non-Mac users:
Function
Mac
Non-Mac
Copy
Command-c
Alt-c
Cut
Command-x
Alt-x
Paste
Command-v
Alt-v
Select All
Command-a
Alt-a
Find
Command-f
Alt-f
Find again
Command-g
Alt-g
Save (i.e. accept)
Command-s
Alt-s
Undo
Command-z
Alt-z
However, as of 3.2, you can switch these shortcuts to the Control key in Windows by opening the Preferences window and choosing one of either: "swap control and alt keys" or "duplicate control and alt keys" (from the "general" category). The balloon help explains it.