When is a speaker not just a speaker? When it's a studio monitor. To the average person, there might not seem to be much difference, but music professionals know better. When you're working in the studio, you need to be able to hear your sound with complete clarity to make sure the recording will sound great even on high-fi audio systems. Studio monitors are designed for that purpose—when every note is crucial and every vocal has to be just right, you need a playback system that reproduces every nuance of the sound.
Studio monitors come in two varieties: powered (or active) and unpowered (passive). When making your decision, it's important to know the difference. Powered monitors have an onboard amplifier, which enables them to perform simply by being plugged in to a power outlet and an audio source. That makes powered monitors easy to set up, so they're a good choice for a hobby studio or for audiophiles looking to get the best sound quality they can when listening to their favorite tunes. Unpowered monitors, on the other hand, need an external amplifier to work. If you're a discerning pro, you might prefer unpowered monitors for the ability to use the same amp head in the studio that you use onstage. An unpowered studio monitor also helps to minimize interference and buzz, since there are no electrical power components in the monitor's cabinet itself.