Most rock listeners want more bass, while many acoustic music listeners will prefer more accurate midrange frequencies instead. For film sound tonal accuracy is less important. Output power determines how much power the amplifier can provide to speakers without damaging them. A larger output power means a louder speaker system. Of course you may not be that interested in a high output power if you live in a dorm or an apartment.
Impedance is the resistance to current flow from amplifier in a speaker.
This value is measured in Ohms. It doesn't determine high or low quality but you should choose amplifier and speakers that are close in impedance rating. Speaker's sensitivity tells you how loud the speaker is per 2.83 Volts input. The higher the sensitivity, the weaker the amplifier you'll need to drive them. This is the ratio of speaker's volume to the underlying static of electronic circuit.