Straight-line depreciation splits the depreciable base (cost minus salvage value) evenly across the useful life, so the deduction is the same dollar amount every year. Declining balance and sum-of-the-years'-digits are accelerated methods that front-load more of the deduction into the early years, when many assets lose value and productivity fastest.
All three methods deduct the same total over an asset's life — the depreciable base — only the year-by-year timing differs. Financial-statement (book) depreciation is also not the same as tax depreciation, which in the U.S. generally follows the IRS's MACRS percentage tables rather than any of these formulas.