Simply put, a dynamic web page is one that changes depending on what is happening. Lets say, you want to have a picture which changes depending on what time of day it is, the time or year, or any other set of circumstances... Which a scripting language (such as JavaScript or PHP) and a database (MySQL or mSQL, or any sql or odbc bases system) you could see what time it was and load a different picture for each time of day.
That is a simple explanation, as much more can be done! We have successfully provided to our clients dynamic WEB applications... applications which enhance their site, provide ways to easily update information, and encourage visitors to keep coming back, again and again.