ETFs (Exchange Traded Funds) are investment funds that trade on an exchange like a stock. Each share contains a complete basket of assets — the 500 companies of the S&P 500, the ~1,500 of the MSCI World, a sector, bonds — so a single purchase gives you diversification that would take dozens of trades with individual stocks.
Most are index funds: they replicate an index automatically, with no manager picking stocks. That automation is the key to their minimal cost and their success: over the long run, few active managers beat the index after fees.
The issuer buys the index's assets and issues shares that trade on an exchange. An institutional creation/redemption mechanism keeps the market price glued to the basket's real value. For you, the experience is identical to buying a stock: an order at your broker, execution in seconds, real-time pricing.
Two decisions define which ETF you buy:
Unlike traditional Spanish index mutual funds, ETFs do not allow tax-free switching: selling one ETF to buy another crystallises a taxable gain. That is their big local disadvantage versus funds. In exchange, ETFs are usually cheaper and trade instantly. Dividends from distributing ETFs are taxed like any other dividend.
They track the same thing, but the ETF trades in real time and is usually cheaper, while the Spanish mutual fund allows tax-free transfers. The choice depends on whether you value cost or tax flexibility more.
Distributing ones do, in cash, at the frequency stated in the prospectus. Accumulating ones reinvest them automatically inside the fund, deferring taxation until you sell.
European rules (MiFID II/PRIIPs) require a key information document that US-domiciled ETFs do not publish. You buy the European UCITS version of the same index instead — often with the domicile tax advantage included.