A disabled account prevents a user from logging on to 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. In the list of users, a disabled account will have a special icon next to the info icon and a disable date shown, like this:
A deleted account also prevents a user from logging on to the system, but is considered inactive. An account may be deleted when it is expected to not be needed again. In the list of users, a deleted account will not be shown, unless you use the Account Status filter. When you do, it will be listed with a special icon next to the info icon and a delete date shown, like this:
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.