Beasts or creeps are one of the most incessantly annoying and useful thing in the game. Creeps, in general, are any species which finds itself unallied to any of the many sides. For example, the Murloc, a violent race found in swamps and all sorts of water, is a creep, as are Forest Trolls. There are a number of goods which can be gained from these things.
The first, of course, is experience. For heroes, this is one of the most important things you can find. This allows you to level up quickly in multi-player games, as well as get to the top possible level quickly in the campaign. The second is money. Each creep which dies gives you some gold and a chance at a special item, which, if you deem it useless (which should be rare, unless all your heroes are completely decked out and it's a potion) then you can sell it for some extra cash.
Below is a link to the bestiary and the different groups of creeps you can expect to find within the game.
 Creep Types
There are many types of creeps in the Warcraft universe, but many can be placed into certain groups. Below are a few examples of creep types and what to expect from them.
A few facts about creeps. Many have special abilities, such as certain types of Murlocs or Centaurs. Also, most creeps give gold and some give items when they die. Expect to find them also guarding something important.
Information on the different kinds of beasts in Warcraft can be found here.