From a practical standpoint, the main difference is that ETF's are essentially a fixed "basket" of securities, similar to unit investment trusts, whereas closed end funds are actively managed. This allows the expenses of ETFs to be kept very low. (Closed end funds typically have high expenses that are not publicized nearly as widely as the expenses of open end funds.)
Per a previous discussion in this forum, ETF's apparently also have a provision that allows a (large) shareholder to exchange shares for shares of the underlying securities. This guarantees that the market value of the ETF will very closely track its net asset value -- not develop a premium or discount the way that shares of closed end funds do.