NAV (Net Asset Value)

Net asset value is the per-share value of a fund's assets minus its liabilities. A mutual fund transacts at NAV, calculated once per day. An exchange-traded fund trades on an exchange at whatever price buyers and sellers agree, which can sit above NAV, called a premium, or below it, called a discount. A premium or discount describes the gap between market price and net asset value at a moment in time. It does not by itself tell you whether the fund is expensive or cheap. Discounts commonly arise from stale pricing in the underlying holdings, from thin trading, or from the fund and its holdings being two loosely linked pools of liquidity.

A mutual fund is bought and sold at NAV itself, struck once a day after the close. An ETF is not. It trades all day at whatever price buyers and sellers agree, and that price can sit above NAV, called a premium, or below it, called a discount. A discount is not a coupon. It usually says something about the plumbing: the underlying holdings may have last traded hours ago in a market that has since closed, the fund may be thinly traded, or the mechanism that keeps price and value together may not be working tightly that day. Whatever caused the gap can still be there when you go to sell.

Further reading: Wikipedia