I thought it might be interesting to expand a bit upon the "clerical error" mentioned above.  There is an interesting bit of irony involved, for sure. The tiger's name should have been "Thirsty."  In fact, the tiger DOES become increasingly thirsty (almost to the point of death) as he survives on the ocean with Pi.

Let's look at the humorous way that Richard Parker gets his name.  A hunter named Richard Parker is hired to kill a deadly panther.  The hunter uses a goat for panther bait.  Instead of finding the panther, the hunter finds a tiger with her cub.  However, instead of running for the food, the tiger cub runs to the water to drink!  This is why Richard Parker names the tiger cub "Thirsty."

The reader comes to understand in Life of Pi that Richard Parker, the Bengal tiger which survives life at sea with Pi is "so named because of a clerical error." (Ch 48) The significance of the name itself creates conflict as the hunted becomes the hunter. As Pi's father was transporting zoo animals when the tragedy occurred, it is conceivable that there would be animals in the story. 

The name Richard Parker was chosen by Yann Martel after he learned of The Mignonette which sank in 1884. The cabin boy was unfortunately the victim of cannibalism by the other survivors. After surviving the incident, the captain of the Mignonette was tried for murder - of Richard Parker, the cabin boy - setting a precedent in the British legal system.  Martel also reveals other Richard Parkers from his research. There's Edgar Allen Poe's Richard Parker, created long before the sinking of the ship but with surprising similarities to the "real" boy. There's also another Richard Parker from another doomed ship from 1846. All these boys are victims and Richard Parker in Life of Pi is also potentially a victim, according to Martel.

Whether Richard Parker is a figment of Pi's imagination or a real tiger, he helps Pi survive his 227 days at sea. Pi's own name created conflict to the point that he only goes by the name Pi, not Piscine, which he goes to great lengths to explain. By using a person's name, Pi can give Richard Parker human qualities, despite his initial fear of him, easing the solitude that haunts him. "It is true that those we meet can change us, sometimes so profoundly that we are not the same afterwards, even unto our names" (ch 5)

What does the tiger represent in Life of Pi?

An answer as to what the tiger represents will vary according to an individual's reading of Life of Pi. Some readers, for instance, will feel that the tiger represents fear. Others may believe that it is a symbol of unrestrained animal nature. In the latter case, it is ironic that the tiger is given the prosaic name of Richard Parker, which suggests an everyday man from an English-speaking country.

One answer which combines well with both the above, as well as many other responses to the book, is that the tiger represents Pi's unconscious mind. In this case, one might say that it is Pi's elemental fear that is symbolized by the tiger. The unconscious is the unrestrained, animal side of the mind, and Pi is forced to allow this side of his mind free rein when he is away from the comfort and protection of civilization. He must do what is necessary to survive, including killing anyone who is a threat to him.

In the third part of the book, Pi tells an alternative story, in which he kills and eats the murderous chef who has killed his mother. The investigators note that Pi plays a similar role in this story to the one Richard Parker plays in the first, giving further weight to the idea that the tiger represents Pi's unconscious mind, the part of the human that is purely animal.