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 2015/2016. With most savings accounts, if you are a tax payer the tax man gets a cut from the interest you earn, that's why it's reassuring to know that with a cash ISA all the interest you earn, you keep.
* Please note that the minimum age to apply for a Flexclusive ISA is 18.