Depreciation refers to the reduction in value of fixed assets over their useful life. Fixed assets include property, plant, equipment, and machinery used in a business for over one year. Depreciation is calculated through depreciation expense and accumulated depreciation accounts to allocate the cost of the fixed asset over its life. There are two main methods for calculating depreciation - straight-line depreciation which uses equal amounts over the asset's life, and other methods which may use accelerated amounts over time.