Teeth naturally yellow with age, and coffee, tea, and tobacco speed it up. Many DIY fixes such as charcoal powders are abrasive and wear away enamel, which ironically leaves teeth looking duller over time.
Professional whitening uses controlled gel that lifts stains from within the enamel without harming it. It will not change the shade of crowns or fillings, so a quick consultation first ensures you get an even, natural result.