Here is a site that shows a video to explain:

http://hyperphysics.phy-astr.gsu.edu/hbase/class/phscilab/vernier.html

This is a pretty cool site that lets you slide the caliper's jaw and see how it would read:

http://www.phy.ntnu.edu.tw/java/ruler/vernier.html

Another one, but no Java:


http://www.rit.edu/~uphysics/VernierCaliper/caliper.html