where individuals pool money to purchase short term low-risk financial instruments
-- in short, a bank
or brokerage account
that pays higher interest
than a standard account because the bank
can play a bit more with your cash and you agree not to withdraw too often.
Once, a money market account might have meant a meaningful return on investment, but today it's often the difference between a bank account that costs you money (either in fees or inability to keep pace with inflation) and one where you barely stay ahead. A typical bank MMA, insured by the FDIC, is currently paying anywhere between 1 - 3% with minimum deposits of around $2000. Internet banks, brokerage houses, and 401(k) funds (not all FDIC insured) pay more money (as much as 5%) as do MMAs with higher minimum deposits (say $50,000).
(Numbers vary, typically in response to the Prime Rate)