www and mysql (since Mac OS X 10.5 (Leopard) _www and _mysql) are user accounts contained in the standard installation of Mac OS X. MAMP / MAMP PRO doesn't create nor configure them in any way. The difference to the regular user accounts is that either www nor mysql don't have home directories and cannot login or use a shell. Those users are only allowed to execute already installed executables that every user is allowed to execute.