A cash ISA allows UK residents aged 16 or over to save up to their annual ISA allowance tax-free with one provider each tax 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.
Remember, in order to use the remainder of your tax-free annual ISA allowance for 2016/2017, the money needs to be put into your account before midnight on 5 April 2017.