Depreciation is a mathematical calculated, annual deduction that the IRS allows for the wear and tear of business property - which includes income producing property and equipment used for business, that has a useful life of more than one year.
Depreciation is based on cost, use and category and is used to decrease a taxpayer's tax liability.
It would take 3 or 4 blog pages to fully describe depreciation. You can view the tax codes on depreciation at www.irs.gov - keyword: depreciation