ISA stands for Individual Savings Account. ISAs are a way of holding savings or investments without paying personal tax on interest received, or on the growth of the investment.
The key difference between an ISA and a standard savings account is the tax treatment on interest received. All interest earned from a Cash ISA is free of personal tax, for as long as those savings stay within an ISA. For standard accounts, basic-rate taxpayers can earn up to £1,000 interest per year tax-free under their Personal Savings Allowance (£500 per year for higher-rate taxpayers, no allowance for additional-rate taxpayers), but any interest earned above that amount will be treated as earned income and could be subject to the payment of tax.
Annual contributions are limited to £20,000 per individual, and any unused allowance can’t be carried over to the following year, but there is no limit on the total ISA balance that can be reached. This means that for Cash ISAs, interest is tax-free regardless of the account balance.