What is SQL? What is MySQL? What is PostgreSQL?

Many computer programs, including web-based programs like blogs, photo galleries and content management systems need to store and retrieve data. For example, blog software need to store the posts (ie, articles) you write, and retrieve them when a visitor goes to your site. Similarly, photo galleries store information about their pictures (for example, for sites that allow users to rate the photos, the numerical rating for each picture is stored in a database).

Instead of reinventing the wheel and implementing their own system of storing and retrieving data, these software simply use the specialised database programs I mentioned earlier.

To make it easy for other programs to access data through them, many database software support a computer language called "SQL" (often pronounced as "sequel"). SQL was specially designed for such a purpose. Programs that want the database software to handle the low-level work of managing data simply use that language to send it instructions.

There are many databases that support the use of SQL to access their data, among them MySQL and PostgreSQL. In other words, MySQL is just the brand of one database software, one of many. The same goes for PostgreSQL. These two databases are very popular among programs that run on websites (probably because they are free), which is why you often see one or both of them being advertised in the feature lists of web hosts, as well as being listed as one of the "system requirements" for certain web software (like blogs and content management systems).