For accounting purposes, depreciation indicates how much of an asset's value has been used up. For tax purposes, businesses can deduct the cost of the tangible assets they purchase as business expenses; however, businesses must depreciate these assets in accordance with IRS rules about how and when the deduction may be taken based on what the asset is and how long it will last.
Depreciation is used in accounting to try to match the expense of an asset to the income that the asset helps the company earn. For example, if a company buys a piece of equipment for $1 million and expects it to have a useful life of 10 years, it will be depreciated over 10 years. Every accounting year, the company will expense $100,000 (assuming straight-line depreciation), which will be matched with the money that the equipment helps to make each year.
Currency and real estate are two examples of assets that can depreciate or lose value. During the infamous Russian ruble crisis in 1998, the ruble lost 25% of its value in one day. During the housing crisis of 2008, homeowners in the hardest-hit areas, such as Las Vegas, saw the value of their homes depreciate by as much as 50%.