There are two types of ISAs (or individual savings account) - cash ISAs and stocks & shares ISAs. Both are designed as tax-efficient ways to save money.
The interest earned on savings in cash ISAs are not taxed. Stocks and shares ISAs, which are also tax free, are an investment product, rather than a savings product and are linked to the stock market where the same basic rules of investment apply. Because of the tax efficiencies there are strict limits on what you can pay into an ISA each year.
Cash ISAs can take many forms. Most are easy or instant-access accounts but occasionally the rate is fixed.