This page discusses some difference of keysets and their function in Apple Keyboards and IBM PC keyboard.
Technically, these keys send the same scancode (key signal):
However, their use by Mac ＆ Windows are different.
The Control Key in Windows is mostly used for keyboard shortcuts. For example, Copy is 【Ctrl+c】, Paste is 【Ctrl+v】. (On the Mac, Command key is used for keyboard shortcuts. Copy is 【⌘ Cmd+c】, Paste is 【⌘ Cmd+v】)
The Control key under Mac is rarely used. When used in applications, it often function as a modifier in combination with other modifier keys. For example, 【Ctrl+⌘ Cmd+z】 in iTunes will iconify the window. Under OS X, the Control is also used for some Emacs's keybinding. For example, 【Ctrl+f】 moves cursor forward.
The Alt key in MS Windows has 3 different uses:
The ⌥ Opt key (often labeled option, ⌥, alt) is used to type special characters, such as “• ™ ©”. For example, 【⌥+8】 will type the bullet character “•”. 〔☛ Mac OS X Keyboard Viewer and Unicode〕
The ⌥ Opt key is also used as a modifier key together with the ⌘ Cmd key. For example, in 【⌘ Cmd+⌥ Opt+h】 invokes the command 〖Hide Others〗 in all Apple applications.
Under Microsoft Windows, the key invokes the Start menu under Microsoft Windows. When pressed with letter key, it invoke a shortcut. For example, 【❖ Win+d】 gets you the desktop. 〔☛ Windows Logo Key Keyboard Shortcuts〕
Under Mac, the Command key (labeled “⌘” or “cmd”) is used as a modifier key to invoke shortcuts. For example, 【⌘ Cmd+c】 is Copy. It is similar to MS Window's use of the Ctrl key.
The ▤ Menu key, also called Application key, exists on PC keyboard only. Typically only on the right side. It is used for Context Menu. The key's function is USUALLY the same as pressing mouse's right button, or pressing 【⇧ Shift+F10】 in most applications.
The primary return/enter key is labeled differently on PC keyboard or Apple keyboard. (this key is different from the Enter ↵ on numberic keypad.)
On PC keyboard, the main enter key is labeled Enter ↵.
On Apple's keyboards, the main enter key is usually labeled Return ↩ or just by a symbol.
The difference between Apple and PC here is just labeling.
On the Mac, the Return ↩ key and the numeric keypad Enter ↵ key usually serve the same purpose, but sometimes different. For example, in Mathematica since at least version 2 (early 1990s), the Return ↩ key is used to insert a line break, but the numeric keypad Enter is used to execute the code in the current cell (【⇧ Shift+Return ↩】 does the same). On many chat clients such as Adium and Colloquy (IRC client), user can set it up in so that the Return will insert a line break while the Enter key will send out the message.
On PC keyboards, there is a ⌫ Backspace key on the main section.
On Apple keyboards, this key labeled “delete” or “⟵” (LONG LEFTWARDS ARROW).
When in text editing, the ⌫ Backspace is used the same way in both OS X and Windows. It delete texts to the left of the cursor. However, MS Windows also use the key to “go back”, such as going to a previous visited page in a browser. Only in recent years, Mac OS X's browsers also started to do the same.
The ⌦ Delete key is located in the ↖ Home/↘ End key cluster. This key sometimes labeled “Del”.
On Mac, this key is labeled “delete” or ⌦.
On Windows, ⌦ Delete is often used to delete things. For example, on desktop, select a file, pressing ⌦ Delete will put the file to the “Recycle Bin”. In a text editor, the key will delete to the right of the cursor.
On the Mac, from early 1990s to 2006, the ⌦ Delete key is almost never used, even in text editors. When there is a function assigned to it, it is used to delete to the right of the cursor.
On PC keyboards, there is the Insert key (sometimes labeled Ins). Apple keyboards doesn't have this key. But, in the same location, it has a key labeled Help.
I think the difference is NOT just labeling; they may actually send different key signals, but the OS or keyboard driver software may interpret them so to user it's the same.
The PC has these keys: PrtScn/SysRq, ScrLk, Pause/Break.
These keys have old history back to the 1980's (or earlier) and in general are not used since 1990, except on Windows the PrtScn key is for screenshot. These keys are basically never used in Mac (as i know of from 1992 to 2007), except that in recent years with OS X, Apple started to use F14 and F15 to decrease/increase the display's brightness.
Apple's keyboards do not have these keys. On some Apple keyboards, they have F13, F14, F15 instead. “Apple Keyboard with Numeric Keypad” of 2009, has F16 to F19.
The {PrtScn ScrLk Break} are different keys than {F13 F14 F15}. They do not send the same signals. In any case, when either PC keyboard or Apple keyboard is plugged in on a Mac running Mac OS X 10.4, these keys have the same behavior (could be due to operating system).
On the Numeric keypad. Apple's numeric keypad added a equal = key. (PC's + key takes 2 key spaces) Because of this change, the arrangement of the arithmetic operator keys are a little different.
Also, the “Number Lock” key on PC keyboard is “Clear” on Apple keyboard. Apple keyboard does not have Number Lock toggle. (a excellent change.)