A fixed deposit earns compound interest. The maturity amount is A = P × (1 + (r ÷ f) ÷ 100)^(f × t), where P is your deposit, r the annual rate, f how many times a year interest compounds (banks usually compound quarterly), and t the tenure in years. More frequent compounding earns slightly more.
Senior citizens usually get a higher rate. Interest on FDs is taxable as per your income slab, and banks deduct TDS above a threshold — the figures above are before tax.