ISAs, or Individual Savings Accounts, offer completely tax free investing and withdrawals. There are two main typyes of ISA, Stocks & Shares ISAs and Cash ISAs, plus additional subsets (Junior ISAs, Lifetime ISAs, Innovative ISAs).
You pay income tax as normal on any money you place into an ISA, but there is no tax at all on any growth, income or withdrawals.
SIPPS & PENSIONS
SIPPS, or Self Invested Personal Pensions, offer a full rebate of your income tax on money invested. The terms are often confused, but all SIPPs are Pensions and today most pensions are SIPPs, in that they offer you a degree of investment choice.
You don't pay any income tax on money you place into a Pension/SIPP and there is no tax at all on any internal growth or income. 25% of withdrawals are tax free with the balance taxed as income.
GIAs, or General Investment Accounts, are standard taxable investment accounts. You pay income tax as normal on any money going in and on any income generated. There is also Capital Gains Tax on any growth, payable when taken.
You can use (where available) your income tax and particularly your capital gains tax allowances to offset against these, but the use of your ISA & Pension/SIPP allowances first is simpler.