Behind the scenes, mutual funds and ETFs are very similar. Both can vary widely in purpose and policies, which is why understanding the prospectus before investing is so important.
Since both mutual funds and ETFs cover a wide range of choices, any discussion of management, assets, or expenses when discussing the differences between the two is inaccurate. Mutual funds and ETFs can both be either managed or index-based, high expense or low expense, stock or commodity backed.
Method of investing
When you invest in a mutual fund, you typically set up an account with the mutual fund company and send your money directly to them. There is often a minimum initial investment required to open your mutual fund account. Mutual funds sometimes, but not always, have a load, which is a fee that you pay either when you put money in or take money out.
An ETF is a mutual fund that is traded like a stock. To invest, you need a brokerage account that can buy and sell stocks. When you invest, you pay a transaction fee, just as you would if you purchase a stock. There isn't really a minimum investment required as there is with a traditional mutual fund, but you usually need to purchase whole shares of the ETF. There is inherently no load with ETFs.
Mutual funds and ETFs are usually taxed the same. However, capital gain distributions, which are taxable events that occur while you are holding the investment, are more common with mutual funds than they are with ETFs, due to the way that ETFs are structured. (See Fidelity: ETF versus mutual funds: Tax efficiency for more details.) That having been said, in an index fund, capital gain distributions are rare anyway, due to the low turnover of the fund.
When comparing a mutual fund and ETF with similar objectives and expenses and deciding which to choose, it more often comes down to convenience. If you already have a brokerage account and you are planning on making a one-time investment, an ETF could be more convenient. If, on the other hand, you have more than the minimum initial investment required and you also plan on making additional regular monthly investments, a traditional no-load mutual fund account could be more convenient and less expensive.