Most sheets of stamps – from the later Victorian ones until around the end of World War Two – had control figures in the margins. For example the penny stamps shown in this picture have the control C38. This means that the stamp was printed in 1938 and was from the third printing plate made – plates A and B would have worn out by this stage and been replaced by plate C.
There is also a number printed in much smaller figures. - in this case 16- which is the cylinder number. The plates were used to make cylinders and these were numbered for quality control purposes.
Of course, some controls are rarer – and more expensive - than others. And there are other variations: in some cases the perforations continue below the stamps right up to the margin of the paper.
For me, the real reason for collecting controls is that they are a bit of fun and they are not horribly expensive. And they look awfully pretty when they're mounted in an album.
See also Hints on collecting
The Five Pound Orange – a jewel of many collections