Account creation is done through Special:CreateAccount. It is possible, through the configuration setting $wgGroupPermissions, to disable account creation or to limit it to sysops; see Manual:Preventing access. There are three types of account creation, each with a separate log event action type:
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 AuthPlugin has been superseded by AuthManager.