Can someone explain db sensitivity?

Generally speaking, sensitivity is measured by driving one watt through the speaker and measuring the relative volume from one meter away. The "loudness" is measured with a SPL meter and indicates how sensitive the speaker is. More efficient designs will have 90+ specs and are easy to drive, making pretty well any amplifier suitable. The most efficient speakers are horn speakers, like the Klipsch's. From there, it's all over the map, whether the design is bookshelf, floorstanders, or panels. Bottom line, find a speaker you like and then find the right amp to drive it.

The higher the sensitivity, the louder a speaker will play with the same input. This loudness is measured in dB. A difference of 3 dB is twice as loud. So a 90dB speaker will play twice as loud as an 87dB speaker with the same input.

The "what is good" question is debatable. Everthing else being equal, a higher sensitivity is better because the amp doesn't have to work as hard to produce the sound level you want.

However, some will say that in order to achieve this high efficiency, compromises must be made in the speaker design so lower efficiency speakers and more powerful amps are the way to go. Others will argue that low power amps sound better than high power amps so higher efficiency speakers and less powerful amps are the way to go.

Note to Herman: A difference of 3 db is twice the power, not twice the loudness. In order to double the loudness you need 10 times the power which is 10db. You need to send twice as much power to the 87 db speaker to achive the same loudness as the 90db speaker.