if u set "Account activation" as "by email" in the settings, then users must activate their accounts by clicking on link in received email.
sometimes when visitors register, they provide wrong/fake email and never receive activation link. In this cases, their "not activated" membershit will last as long as u set it in the settings, and nobody else will be able register with the same name/email address. After the exparation time, those "not activated" accounts will be deleted from database and visitor will be able again try to register with the same name/email.
