Accelerated depreciation is the allocation of a plant asset's cost in a faster manner than the straight line depreciation. Compared to straight line depreciation, accelerated depreciation will mean 1) more depreciation in the earlier years of an asset's life and 2) less depreciation in the later years of the asset's life. [Note that the total amount of depreciation over the asset's life will be the same regardless of the depreciation method used.] Hence, the difference between accelerated depreciation and straight line depreciation is the timing of the depreciation.
The U.S. income tax regulations allow a business to use accelerated depreciation on its income tax return while using straight line depreciation on its financial statements. For profitable corporations this will likely result in deferred income tax payments being reported on its financial statements.