IMPORTANT NOTE: You do NOT have to register to read, post, listen or contribute. If you simply wish to remain fully anonymous, you can still contribute.
ISAs are unique to the UK - although many countries have similar tax-advantaged schemes: Ireland doesn't.
Put simply. You can invest up to £20k every year in an ISA - using it to buy stocks and shares (or other financial products/deposits).
Any profit made is free from tax of any kind - anywhere/any type. Small amount of research is needed, or advice (your bank will organise it) - but the risk is only where YOU decide to invest.
Be careful though - although it's authorised by HMRC, and has been operating satisfactorily for many years. You must be a UK resident......DYOR