depreciation

Think about buying a 30,000 car you expect to drive for ten years. It would feel wrong to call the whole 30,000 a 'cost of this year' — you will be using the car for a decade, not just now. It would feel equally wrong to pretend the car costs nothing once you have paid for it, since each year of driving wears it out a little. Depreciation is the accountant's way to split the difference: spread the car's cost across the years it actually helps you, recognizing a fair slice of it as an expense each year.

Depreciation is the systematic allocation of the cost of a tangible long-lived asset over its useful life. It is driven by the matching principle: an asset earns revenue over many years, so its cost should be matched against revenue over those same years rather than dumped into one. The amount spread out is the asset's cost minus its salvage value (what you expect it to be worth at the end). Various methods exist for the spreading — straight-line, units-of-production, declining-balance — but all do the same job. Importantly, depreciation reflects wear, age, and obsolescence; it is not an attempt to track the asset's market price.

Depreciation matters because it shows up as an expense on the income statement every year, quietly lowering reported profit, and because it builds up in accumulated depreciation, lowering the asset's book value on the balance sheet. The single most important thing to understand is this: depreciation is a non-cash expense. The cash left the business when the asset was bought; the yearly depreciation entry moves no money at all — it is purely an allocation. That is why a profitable company can have depreciation dragging down its net income while its cash balance is untouched.