With ING or HSBC's online savings account, the Bank is paying you a static rate of return on your money. The interest rate that these banks pay is less than their cost to borrow from other banks, which is why the interest rate will change when the Fed raises / lowers their target rate. With a Money Market account you are getting a true market rate of return on your deposit, which is why minimum balances may be higher than the online saving accounts. Because your deposit is invested in slightly less liquid financial instruments (municipal bonds, corporate bonds, etc) withdrawals are limited because of the transactional costs associated with liquidating those investments.