An ETF holds a portfolio and issues shares against it. Those shares trade all session at a price set by supply and demand, while authorised participants create and redeem blocks of shares to keep that price tethered to the value of the holdings.
The structure explains the appeal: one order gives exposure to five hundred companies, an entire sector, a commodity or a bond ladder, usually at a fraction of the fee an equivalent mutual fund charges, and with no minimum beyond the price of one share.
Read what is inside before you read the ticker. Leveraged and inverse ETFs reset daily and decay over longer holding periods, commodity ETFs that hold futures carry roll costs, and a thinly traded ETF on a liquid index still costs you a wide spread.