what is the difference between a balanced and unbalanced speaker or mic ?

First off, there is no such thing as a balanced speaker. Only electronics are balanced. There is no such thing as a balanced speaker cable either...though there are a few speaker cables that use XLR balanced connectors. Also, don't confuse Speakon speaker connectors with XLR...they look somewhat similar, but they are very different. And speakers that have amplifiers on board (many sound reinforcement speakers are set up this way) use XLR inputs, but this is for the amplifier. The actual connections between the speaker and internal amp are not balanced.

Now, as for electronics...balanced refers to the way the signal passes between and thru the electronics. In consumer electronics, most connections are made via RCA connectors. These are known as single ended connections. The signal is carried on one conductor and the ground is on the other.

With higher quality consumer gear and nearly all professional products, the signal is carried by a three wire conductor. In a balanced signal, the signal is split and one side carries the signal 180 degrees out of phase to the other. When the signal gets to the end, the two out of phase signals are recombined and any noise that is injected into the line is cancelled out. Balanced lines are particularly good for long runs, which is why you'll see nearly all studios and professional venues wired using balanced connections.

Most microphone cables and microphone preamps are balanced because a mic is one area where noise can really intrude. Also, microphone cables tend to be fairly long.