create— a new user creates their own account.
create2— another user account is created by an existing user.
autocreate(up until MediaWiki 1.26) — a new account is autocreated using AuthPlugin. AuthPlugin has been superseded by AuthManager.
It is possible, through the configuration setting $wgGroupPermissions, to disable account creation or to limit it to sysops; see Manual:Preventing access#Restrict_account_creation (which describes how a sysop may create another user's account).