Speakers on a stereo system are designed to 'sound good' and spread sound over a fairly wide physical field. This is done so that everyone in the room can hear the sound resonably well. The problem with this is that the sound you hear is not only coming from the speaker itself but also reflecting off ceilings and walls. Thus you are hearing the reverberation of the room too. Speakers also tend to emphasise or de-emphasise certain frequencies.
Monitors are designed to give you an accurate representation of sound in a more narrow physical field. This is what is known as 'near field' I believe. You are not hearing the sound as it bounces off everything in your room but mainly as it comes out of your monitor. It also produces a relatively flat frequency response which just means it does not favour any specific frequencies. This gives a more accurate picture of whats going on sonically.
I remember someone once saying 'Speakers are built to hide the flaws in audio. Monitors are built to expose the flaws so you can fix them.'
