When using the MyISAM storage engine, MySQL 
uses extremely fast table locking that allows
 multiple readers or a single writer. The biggest
 problem with this storage engine occurs 
when you have a steady stream of mixed updates 
and slow selects on a single table. If this is a 
problem for certain tables, you can use another
 storage engine for them. See 
Chapter 14, MySQL Storage Engines and Table Types. 
MySQL can work with both transactional and 
non-transactional tables. To be able to work smoothly 
with non-transactional tables (which can't roll back 
if something goes wrong), MySQL has the following rules 
(when not running in strict mode or if you use the
 IGNORE specifier to INSERT or UPDATE).