I'll explain the simpliest one first: xclock functions exactly as you'd expect it would. It ticks off the seconds, minutes and hours in a small window.
No amounts of clicking or typing in xclock's window will affect it--that's all it does. Or is it? In fact, there are various different options you can give to the program to have it act in different ways. For instance, xclock -digital will create a digital clock. xclock -update 1 will create a second hand that moves every second, while -update 5 will create a second hand that moves every 5 seconds.
For more information on xclock's options, consult its manpage--man xclock. If you're going to try running a few of your own xclocks, you should probably read Section .