A disabled account prevents a user from logging into the system, but the account remains active. An account may be disabled simply because the testing window has passed, so the account has its access removed until the next time it is needed.
A deleted account also prevents a user from logging into the system, but is considered inactive. An account may be deleted when it is expected to not be needed again.
Regardless of category, user accounts are never completely deleted from the database. This is because every action performed in the system by a user is logged. All actions are tied to a user account. Completely removing the account would orphan the actions and make audits impossible.