open loop amplifier is that in which there is no feedback.usually the bandwidth of open loop amplifiers is very low(eg opamp 741).in order to make an amplifier usable for practical purposes ,it needs to be used in closed loop mode,which is essentially negative feedback.when u use negative feedback it means that u are willing to sacrifice a portion of the gain of your amplifier for the sake of improved bw,noise performance ,input resistance and o/p resistance etc.i suggest u refer to integrated electronics by millman and halkias for further information.
regards
amatnath