The difference is stereo provides the representation of a soundstage in front of you, where sounds are perceived to come from one side or another, or anywhere in between.
Mono provides no illusion of a soundstage -- all sound appears to come from a single point in space, directly in the center of a pair of speakers (unless you mess with the balance control!)
As to one being better - since stereo generally provides a more realistic audio "model" of a sound field, and since we are used to hearing sound coming in different directions, most people tend to prefer it..... however a good mono recording is better than a poor stereo recording, all things being equal.