An ISA, or Individual Savings Account is a ‘wrapper’ in which you can shelter savings and investments from tax. It is not an investment in its own right.
There are significant tax advantages of investing in an ISA- you pay no capital gains tax and no UK income tax on your ISA savings. You don’t need to declare ISAs on your tax return.
Any UK resident aged 18 or over (16 for Cash ISAs) can invest in an ISA and you can withdraw your savings whenever you want to.