A cash ISA allows UK residents aged 16* or over to save up to their annual ISA allowance tax-free with one provider each year. The allowance is £15,240 for 2016/2017. With other savings accounts, you may have to pay tax on the interest you earn, depending on your Personal Savings Allowance. The interest on a cash ISA is tax-free, so all the interest you earn, you keep.
* Please note that the minimum age to apply for a Flexclusive ISA is 18.