More on this is here!!
Entropy = disorderliness
Take a desk for example...All the books are arranged in alphabetical order, the pencils are in the little cup (erasers pointed up), all the pens have caps and are arranged by color of ink. What's more, all the papers are filed (also in ABC order), the stapler, paper clips, etc. are all tucked away neatly in the drawer...... This desk is said to have a low level of entropy, in other words, it's pretty well ordered.
Now somebody comes in, tips the desk over, places it back upright, and throws all the spilled contents back on top. What we have here is, yup you guessed it, a high level of entropy.
What does a messy desk have to do with a blackhole's entropy? Well let's break it down like this. A blackhole is identified by its three intrinsic properties....mass, charge, and spin. Somebody may say well how the hell can a blackhole have entropy when it is nothing more than a spinning gravitational field. (This is equivilant to a messy desk that only has just a book and a pencil......its not able to be messy!!)Where's all the pencils and pens....(i.e. things such as particles to say, ok, this blackhole is a mess because the particles are completely randomly distributed and have no order). Well the answer lies in this line of reasoning, thought up by Hawking I believe.....
Since the event horizon always expands, and the entropy of a closed system will always and inevitably increase (second law of thermodynamics), there must have been a relation there. Since the size of the event horizon is given in terms of the blackholes surface area, then there must be a direct mathematical relation between the surface area and the entropy (Entropy = Surface area of blackhole/4). Now that we established the entropy for any given blackhole, then we may also state that a blackhole has temperature, (study the relationship between temperature and entropy) and hence a blackhole isn't actualy black, but actualy radiates energy in a manner stated in the Hawking-Bekingstein Radiation theory. In other words there is no such thing as a blackhole, only very very "dim holes".
You should really read Brian Greene's Elegant Universe, he goes into detail while I VERY BREIFLY touched on a small piece of the whole story.