Monitors (aka Studio Monitors) have a very flat response. Regular stereo speakers with have parts of the audio spectrum emphasized or de-emphasised to give them more a pleasing sound - more bassy, more trebbly, lower mids, etc. Compared to stereo speakers, monitors can sound harsh.

When you are mixing audio tracks, you want an accurate representation of the sound through flat monitors, otherwise you may be compensating for speaker characteristics in your mix.

Many monitors you find have built in amps, so you just have to input an audio source (like the M-Audio device connected to your computer) and you are good to go.

I have a pair of M-Audio BX2 5" monitors I use that I'm happy with. For listening to music, I added a polk audio sub to round out the bass. When I'm mixing, I turn the sub off.