What does OpenCV's cvWaitKey( ) function do?

cvWaitKey(x) / cv::waitKey(x) does two things:

1. It waits for x milliseconds for a key press on a OpenCV window (i.e. created from cv::imshow()). Note that it does not listen on stdin for console input. If a key was pressed during that time, it returns the key's ASCII code. Otherwise, it returns -1. (If x is zero, it waits indefinitely for the key press.)
2. It handles any windowing events, such as creating windows with cv::namedWindow(), or showing images with cv::imshow().

A common mistake for opencv newcomers is to call cv::imshow() in a loop through video frames, without following up each draw with cv::waitKey(30). In this case, nothing appears on screen, because highgui is never given time to process the draw requests from cv::imshow().

Plain simply, cvWaitKey() sleeps for X miliseconds, waiting for any key to be pressed.

int cvWaitKey(int X);

If a key is pressed, this function returns the ASCII code of key. Or returns -1 if no keys were pressed during that time.

cvWaitKey(0) stops your program until you press a button.

cvWaitKey(10) doesn't stop your program but wake up and alert to end your program when you press a button. Its used into loops because cvWaitkey doesn't stop loop.

Normal use

char k;

k=cvWaitKey(0);

if(k == 'ESC')

with k you can see what key was pressed.