I think some more detail is called for here too, on mutual funds vs ETFs:
When you buy part of a mutual fund, you are giving your money to professional fund managers to invest for you. Mutual funds are often devoted to a single investment strategy (value, growth, index...) or a specific business sector (energy, health care, high technology), or even a specific kind of investment vehicle (stocks, bonds, commodities...).
You pay the fund managers a small percentage of your assets each year (the number you want to look for here is the "expense ratio"). Something on the order of 1%. Sometimes you also pay a fee when you put your money in or when you take it out; funds that do this are called "load" funds, funds that don't are called "no-load" funds.
When you buy into an ordinary mutual fund, it's a similar process to having a savings account: you send the fund money, they use it to buy financial investments. Mutual funds are generally sold and redeemed at par; each dollar you invest in the fund buys a dollar's worth of investments. When you cash out, each dollar of investments they sell is a dollar that goes back into your pocket.
ETFs are similar to stocks. When you buy shares of an ETF, you're buying a piece of the fund from another investor, not putting money into the fund directly. ETFs are often traded at a discount to net asset value. In other words, you pay less than the market price of the investments the fund owns. But that doesn't necessarily make it a better deal, because of course when you want to cash out, you will probably be selling below par as well.
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)