MySQL is a quasi-SQL DataBase management system (RDBMS).

It is small and fast for low loads. It lacks a lot of features that other databasen, noticably PostgreSQL, offer. (PostgresVsMysql)

When should I use MySQL?

When your primary concern is the speed of simple SELECT queries for few concurrent users. Even then PostgreSQL is getting much quicker at these.

When should I not use MySQL?

When you think "It's only going to ever be a small web site...", and there's a chance it could grow into a big one. If at any point you think you might need to change to something else, start with that other system instead.

How can I lessen the impact of having to switch later on?

Use PearDB? in your PHP scripts.

(JARRING CHORD)

(The door flies open and Bill Gates of Microsoft enters, flanked by two junior cardinals. Steve Ballmer has goggles pushed over his forehead. Sam Palmisano (IBM) is just an idiot.)

Gates: NOBODY expects the InfoWorld Article! The chief thing missing from MySQL is subselects...subselects and views...views and subselects.... The two things missing are views and subselects...and triggers.... The three things missing are views, subselects and triggers...and an almost fanatical devotion to row-level transactions.... The four...no... Amongst the things missing from MySQL ...are such elements as views, subselects.... I'll come in again. (Exit and exeunt)

Thanks LeandroDutra? for your edits, but please note that incorrect capitalisation is often the sign of a WikiWord, and shouldn't be edited. The same goes for acronyms that might not be correct but have a link attached to them - if you are going to change them, please create the new link and then link the new page to the old one for reference.