MySql is a traditional "back-end" tool which can store data which can be collected and placed into a page before it is delivered to the browser.
There are many different ways to store data besides an sql database,
cookies, xml or csv files as well as sessions are all valid methods of storing data.
The Sql part refers to the Structured Query Language and is used to manipulate the data in the store.
Most brands of database applications such as Ms Access, Informix, Oracle and of course MySql
use a common basic Sql set but there are differences in each of them to be aware of.
The MySql database server is again a free tool and a very powerful one too, it knocks the spots off the Ms Access database in too many ways to list here, but in all fairness it's features and performance comes out around the same as Microsoft's Sql server, but with a price tag of £0 instead of £600 it has proven to be very popular.
Its not just the initial price that deters site owners from using the Microsoft Sql server but like all of Microsoft's applications it requires a £300 upgrade every couple of years and the painful process of the installation. MySql and Php do not not require an install, you simply copy the new version into a folder and hey-ho its all over, no money, no re-boot, no hassle (why is it Microsoft insist on a re-boot every time you change something?)
MySql is another dbms (Database Management System), or store of data that can be queried and filtered. It is quick, powerful and like most GNU applications it is solid. Widely used for storing and retrieving items in your web site catalogue or information on registered users.