MySQL is a relational database management system (RDBMS) commonly used in dynamic websites. A MySQL database allows you to store and retrieve structured data on a server which can be used to create dynamic webpages such as a bulletin board or guestbook.
MySQL is typically used with PHP, however it can be used by any scripting language that has supporting drivers. Getting and setting data in a MySQL database is pretty much the same as any other RDBMS. The use of SQL (Structured Query Language) or queries as they are also known is used to get data from the database matching only the criteria that is asked for. Its the power and flexibility of SQL that makes database so important in modern day computing and the internet.
A database can be considered a little like a phone directory. When using a phone directory you first look for a name and then filter through the results to find the most relevent match. MySQL is very similar. You store the data with keys or identifiers such as a first name or surname and then you can use SQL to fetch the rows of data. In a web based application this data is then stored in variables and arrays. The arrays of data are then processed to create rows of HTML data which is then passed on to the end user through their web browser.