Depreciation is a way to account for the decrease in value of tangible and intangible assets over time due to factors like wear and tear, damage, obsolescence, or age. There are several methods for calculating depreciation, including the straight-line method, declining balance method, and sum of years' digits method. The straight-line method charges a fixed depreciation amount each year, while declining balance and double declining balance methods charge higher depreciation amounts in early years that decrease over the asset's life.