Depreciation expense is that portion of a fixed asset that has been considered consumed in the current period. This amount is then charged to expense. The intent of this charge is to gradually reduce the carrying amount of fixed assets as their value is consumed over time. This is a non-cash expense; that is, there is no associated cash outflow.
When an entry is made to the depreciation expense account, the offsetting credit is to the accumulated depreciation account, which is a contra asset account that offsets the fixed assets (asset) account. The balance in the depreciation expense account increases over the course of an entity's fiscal year, and is then flushed out and set to zero as part of the year-end closing process. The account is then used again to store depreciation charges in the next fiscal year.
The same concept is used for intangible assets, where the associated expense account is referred to as amortization expense.