Frequency response
Frequency response, measured in Hertz, refers to the range of frequencies that a speaker can reproduce. A loudspeaker will produce a certain loudness when supplied with an audio signal of a given level and a given frequency. Ideally, if you keep the level the same, but vary the frequency, the loudness will stay the same. This will be called a flat frequency response.
The average human ear can detect a maximum frequency range of 20Hz to 20KHz, so look for speakers that can reproduce as much of that range as possible. Keep in mind that there is no standard on how to measure, so you'll need to consider other features when comparing speakers.
impedance
Impedance, measured in ohms, describes how much resistance an amplifier will encounter when trying to drive a given speaker. Most speakers are designed with an 8 ohm impedance to work with almost any home audio equipment. For consistent response, all speakers should have the same impedance; so when replacing a single speaker, make sure it matches the rest of your setup.
sensitivity
Sensitivity is measured in decibels per watt and refers to the volume of audio a speaker can deliver per watt of power. The higher the sensitivity, the less power the speaker needs to achieve high volume. Low sensitivity is approximately 88 dB per watt, while high sensitivity is 100 decibels per watt.
power handling
Power handling capability is measured in watts and expresses the maximum sound signal that a speaker can accept. Powering a speaker with more power than it can handle can damage the speaker, but many speakers with high power handling capabilities actually require a more powerful amplifier to sound their best. For best results, choose speakers that can handle approximately the receiver's maximum output. Keep in mind that there is no standard on how to measure, so you'll need to consider other features when comparing speakers.