Easy to follow explanation on what JTAG is and how to use it to hack your way into hardware devices.
When coding for Arduino you may wish to run certain code once every N milliseconds. For a very simple program like blinking a LED with a 2 seconds interval using the delay() function will be sufficient. But what about multiple bits of code where each require a different interval? Using delay() they will slow each other down, significantly disrupting your program. The solution is the meassure the last time the code parts were executed.