I think, that can answer more simply. If you look Datasheet of any opamp you can see Bode Plote (where represents open loop gain vs frequency and phase vs frequency). Point, where crosses 0 dB and curve of Open-Loop Gain will be GBWP. From it, you can get maximum frequency on given gain(for example if opamp has GBWP = 10Meg, and you will be use it as amplifiers with gain = 10, maximum -3dB freq will be 1 Meg).
UGB - when you amplifier is buffer, ie it has Gain=1. This freq looked at point -3dB.