php allows you to do many things such as bring data to other pages, so for example when you join a website you will enter your username and password, using php this is then taken to the next page where it is placed in the database and if it worked fine then your would get an email etc... this is all php (only mentioning a few things) it also is used in sites such as Yahoo! answers and in almost all content management systems!
A good place to learn it is tizag.com (that was where i learnt about php)
