Within the wrapper there is nothing to report on a tax return, no uk-capital-gains-tax on disposals and no further tax on dividends. The annual subscription limit resets each tax year and unused allowance cannot be carried forward.
Investment choice is restricted to qualifying investments, broadly listed shares on recognised exchanges, funds and bonds. Leverage, shorting and most derivatives are not permitted, so an ISA is an investment wrapper rather than a trading account.
uk-stamp-duty-reserve-tax still applies to UK share purchases inside an ISA, and foreign withholding tax on overseas dividends is generally not recoverable within it.
This is general information about the United Kingdom, not tax advice. Rules change and depend on your circumstances; consult a professional.
Related: uk-capital-gains-tax, sipp-uk, uk-stamp-duty-reserve-tax, uk-bed-and-breakfasting-rule, hmrc