Why are the corner pieces referred to as "rooks" in English? Various sources like those quoted in Wikipedia say that it's a mistake, or old-fashioned, to refer to rooks as "castles".  Why is this, though?  Other languages generally refer to these pieces as "towers" and when the king exchanges places with them, we call it "castling" rather than "rooking".  The pieces look like castles.  The term "rook", on the other hand, seems to come from Persian/Arabic and have a very obscure etymology.  So why is this word preferred?
Some say: 

There will probably other theories about the origin of the name "rook" for this chess piece; however here is the one I believe is best.

IN the origins of the game itself, the game was called Chaturanga and it was not exactly the same as modern chess. The piece we call a rook was considered to be a chariot rather than a castle, probably because of the speed with which it moves. The Sanskrit word for chariot was "ratha". In Arabic it is still referred to as a chariot with the name "rukhkh." When the game spread to Europe, the word "rukhkh" sounded like the Italian word "rocco", which meant "tower." Since the two words sounded alike, the Italian word was used but the meaning changed from chariot to tower.

The piece is still thought of as a tower in European translations. Various European countries use their language's word for "tower" rather than their words for "chariot". In Middle English the concept of tower eventually turned into castle since most castles had towers.

Contributors are welcome to add to this answer any other theories about the name; however they should honor the rule against deleting other answers unless they are obviously wrong. Contributors are also reminded of the rules regarding minor edits.

Other explanations (the ones i believe to be true):

The rook was already known in "Chaturanga", but there, this figure was a carriage and was called "rukh". The war carriages have been a part of the old Indian army until the 5th century. At the time the game came to Arabia the name did not change but the portrayal was simplified. In Europe the English name "rook" remind us to the descent.

But how has the figure changed from a war carriage to a rook?

By the year 1527 Vida, bishop of Albay, published a poem about a chess game between "Apollo" and "Mercury". The rooks were fortifications on the back of an elephant. The European chess players took over the description. Finally they left out the elephant for the normal use.



Notes, I told you what others have said, what you're going to read now is my theory

I am from an Arabic country, Lebanon, and we don't call the bishop "Al Fil", like some website say, at least in my country we don't, maybe some other arabic country did, not us, and we still play Chaturanga (but i don't). We call it Chatarange, it's a French name, since France was the mandate for Lebanon long time ago.

Anyway lets talk about the pieces which is interesting


king = king
queen = queen
knight = horse
rook = castle, yes we call it castle, and it makes sense to castle for
us, we don't rook :)
Bishop = minister, and also makes sense, the Arabs who
played Chess were Muslims you know :) Why European call it Bishop? I
can think of 3 reasons:


Because the bishops were so powerful and involved in political life. Because Christian armies relied on them during the wars, they were the one who blessed a war (Remember the crusaders and the Pop?). And lastly, because they contributed so much to the game (Ruy Lopez, was created by a Spanish priest who later became a bishop!) 


Oh so you think I forgot about Pawns? Wrong! Pawns = Some call them soldiers, and some call them stones or rocks! 


Now my theory: 

It is possible that by translating, a castle, became a rook, after all a castle, is built using rocks. Why didn't the European remove the term "pawn"?

By definition a Pawn is the condition of being held as a pledge against the payment of a loan. All the poor and the farmers were "Pawns".

Remember that Chess was introduced to Europe in mid centuries and only the wealthy played it, so the term pawn has a very significant meaning, the weakest on the board in a Feudal society, that would fight for the king in the hope of being promoted! So they left the name as is for that piece.

Small Edit To My Initial Answer

You might be wondering: Why the hell would Arabs call "Pawns" as "Rocks" in the first place?

That's because, Arabs play a board game called Tables, the game as we play it today, was created in Turkey, (I don't give a damn about what Wikipedia has to say about the history of the game) and the Turks have invaded the Arabic world for a long time, so that's how we got it.

The game is played by stones or rocks, rounded pieces, now they are made from plastic, but not back then. The Tables game is far more popular than Chess or whatever you call it in the Arabic world today.

The way you play the game is by pushing the pieces to the other end of the board, and you win (I'm not going to go over the rules, but that's the idea)

There's also Checkers, which originated in Iraq some believe, and is still more popular than Chess nowadays in the Arabic world.  And the pieces move very similar to Pawns and again were made from rocks. So why creating a "Pawn" shaped piece, when you can use rocks and start playing?

Sources


The history of the chess pieces
Chess Piece history‎: Rook
Why is a rook called a rook?