In case your total income does not exceeds the maximum income chargeable to tax, and you don't want the other person to deduct your tax this Form 15 G is required.For Ex:-Suppose you are a resident individual and you have invested some money in Fixed Deposits/Recurring Deposits with Banks and accumulated interest amount during the year exceeds Rs 10,000 Bank will deduct TDS @10% provided you have provided your PAN Number (otherwise @ 20%).So here in this case as you know that your total income won't exceed maximum amount chargeable to tax you can fill form 15G which states non requirement of deduction of TDS.15G AND 15 H both serve same purpose except for that 15 H is for senior citizens nd 15 G is for all others..