They are known as the fruits of the Sea Devil and the secret to their power is hidden in the Grand Line. Devil fruits are rarely found in the world of One Piece. Although these fruits are disgusting and tasteless, anyone that consumes one would be granted a great power or trait. There are more than 100 types of Devil Fruit and they come in different shapes and colors. It is known for a fact that only one bite is needed to obtain a power of from the Devil Fruit. If you swallow it whole, as Buggy had, the effects are still the same. Also, objects are capable of eating the Devil Fruit seen in objects such as Lasso the dog/gun (owned by Mr. 4) and Funkfreed the elephant/sword (owned by Spandam).
Although the storyline hasn't revealed it, there are certain ways of telling what powers a Devil Fruit would grant you. Shanks and his crew already knew that Luffy's Devil Fruit was the Gomu Gomu no Mi before he consumed it. Spandam mentions that the Devil Fruits he supplied to Kaku and Kalifa were not listed in any catalogs anywhere as to what powers they granted. He also said that Devil Fruits have an aura around them which can help identify them. However, the two Devil Fruits that Spandam obtained had an unknown aura.
Blueno, member of CP9, stated that Grand Line scientists have noted that if a person consumed two Devil Fruits, the person's body would destroy itself. Furthermore, devil fruit powers can be affected by seastone. In the recent chapters, Doctor Vegapunk, a Navy Scientist, researched the effects of the Devil Fruit and seastone. He is also the one that created the method to allow objects merge with devil fruits. It should be noted that many of the fruit names come from Japanese onomatopoeia and vocabulary.