Or, another way of putting it is, when you first signed up with your ISP they gave you a initial email address,
mine for argument sake with VM "is similar to" ph01xxxxxx789@virginmedia,co,uk
(which also happens to be my account number, so how stupid is that)
They also gave me the facility to create up-to 5+ more email address's, any of these 5 would be classed as "Alias" address's.
Because they are created from within your ISP account (if they are used), they are then recognised (as Kitz points out) as part of your account.
My original address being, ph01xxxxxx789@virginmedia,co,uk
One of my alias' could also be = AKA (also known as) oldfogy@virginmedia,co,uk