In terms of the control over your system offered by keyboard commands, pressing Ctrl+Z has the effect of stopping the current program. It suspends the program until you switch back into it or tell it to resume in another way, or manually kill it.
In the same style, pressing Ctrl+C while a program is running will quit it. This sends the program’s process a termination signal, a little like killing it by using the top program. Ctrl+C can prove handy if you start a program running by accident and quickly want to end it, or if a command takes longer than you expected to work and you cannot wait for it to complete. It’s also a handy way of attempting to end crashed programs. Some complicated programs don’t take too kindly to being quit in this way, particularly those that need to save data before they terminate. However, most should be okay.
Ctrl+D is another handy keyboard shortcut. This sends the program an end-of-file (EOF) message. In effect, this tells the program that you’ve finished your input. This can have a variety of effects, depending on the program you’re running. For example, pressing Ctrl+D on its own at the shell prompt when no program is running will cause you to log out (if you’re using a GUI terminal emulator like GNOME Terminal, the program will quit). This happens because pressing Ctrl+D informs the BASH shell program that you’ve finished your input. BASH then interprets this as the cue that it should log you out. After all, what else can it do if told there will be no more input?
Although it might not seem very useful for day-to-day work, Ctrl+D is vital for programs that expect you to enter data at the command line. You might run into these as you explore BASH. If you ever read in a man page that a program requires an EOF message during input, you’ll know what to press.