Of the great many keys used on computer and terminal keyboards in the past (which I listed in my other post), the majority are long gone. But there are still a few keys around -- like Print Screen, and Scroll Lock -- that have old names, but new functions (or no functions). This is my recollection of some keyboard functions from the original IBM PC and several similar computers of the 1980's. Everything I say here is strictly from memory; there are undoubtedly some computers, and some key functions, that I never saw.
Keep in mind that the OS under which everything then ran was not any kind of Windows–like system; it was MS–DOS, which looked like the "Command Prompt" except that it was full–screen. When you start a program, DOS disappears from the screen but is still running; when you end the program, it reappears.
Scroll Lock: This key, when used with some old word–processing programs, would force the arrow keys always to scroll the whole document up, down, left or right, regardless of where the cursor was. Today we have scrollbars instead. Some programs used the key in other ways, or not at all.
Num–Lock: Still works, but now seldom needed. Old keyboards didn't have both a Num–pad and a Cursor–pad; they just had one keypad, and it functioned either way depending on the setting of Num–lock. Now, with both pads, you can keep Num–Lock always on.
The "Center" Key: If you do turn Num–lock off, the Num–pad becomes a Cursor–pad, with 2, 4, 6 and 8 acting as the arrow keys, and so on. But the center key (the 5) has never had any Cursor–pad function, so with Num–lock off it becomes a dead key, as it always did. With an application that allows you to assign your own functions to keys, you could make that key do something; but every time you want to use it that way you'd have to turn off Num–lock first!
Escape: If you messed up the syntax of an MS–DOS command, you could start over by pressing the Escape key. All it did was make DOS ignore that line and start a new one.
The "Control" keys: Combinations like Ctrl–A, Ctrl–B, and so on, are "control characters" which could be typed to perform various functions, or inserted into a document. Most of them are now unneeded since the same functions can now be done without them.
Enter and/or Return: These are really two different, and pretty much unrelated, concepts. On the old terminal keyboards used to access mainframe computers, Enter was one key and Return was another. "Return" means move the cursor (to the next line, or the next field, or whatever); "Enter" means pass the data you've typed to the program for it to act upon. Today's Enter or Return keys sometimes try to combine both functions, making it necessary to use other methods when you need one function without the other.
The "Fn" Key: Originally this key could be combined with any character key, making combinations like Fn–7, Fn–R, Fn–/, and so on. Certain ones were sometimes predefined, and the rest could be defined to do whatever you want (assuming you were running an application with that capability).
Alt + Character Codes: This still works just as it always did. If you hold down Alt while entering (on the Num pad) the numeric code for any character, it's thereby typed. For example, Alt–65 types an upper–case A because 65 is the code for that character. This makes possible the use of non–English letters and many other characters not on the keyboard, if the character is recognized by the current application. Microsoft Word has lots of characters; click "Insert > Symbol > More Symbols" to see them all.
Pause: Originally this key interrupted whatever was running, and jumped into a BIOS routine that would keep looping until you press another key. So it would make the computer "freeze." Today the Windows–type OS's have made that function pretty much obsolete.
Break: This was used to abort an application. MS–DOS had a default Break routine that would interrupt and terminate any program without crashing. Or, if you were writing a program yourself, you could make it respond to the Break key in some other way. Today you can abort an app just by closing a window, so you don't need Break.
Print Screen: Now used for things like screenshots, this key originally triggered a BIOS routine that would send the contents of the screen to a printer, and then let you continue working while it printed. But it could only print text, and it only worked with a "parallel" printer.
SysRq (System Request): This key, on those systems that supported it, would suspend everything currently running, and load a second copy of MS–DOS in a separate part of memory. Then you could use DOS commands or even run another program. When you exit from the second DOS it returns to the first DOS, and to the program that it was running.