A loss is recognized when assets are sold for a price lower than the original purchase price. Realized loss occurs when an asset which was purchased at a level referred to as cost or book value is then disbursed for a value below its book value. Although the asset may have been held on the balance sheet at a fair value level below cost, the loss only becomes realized once the asset is off the books.
One upside to a realized loss is the possible tax advantage. In most instances a portion of the realized loss may be applied against a capital gain or realized profit to reduce taxes. This may be quite desirable for a company looking to limit its tax burden, and firms may actually go out of their way to realize losses in periods where their tax bill is expected to be higher than wished.