Put a rook on d4 and it attacks fourteen squares: the whole d-file and the whole fourth rank. Put it on h1 and it attacks fourteen squares. Put it in the middle of a busy position and the number drops, but on an open board it is always fourteen, and not one of them is diagonal.
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h 1 Article 3.3 of the Laws is one line long: the rook moves to any square along the file or the rank on which it stands. Straight lines, and it stops at the first piece in the way.
The move that looks like an exception
Castling. The rook leaps over the king, which is the only time in a game that a piece passes another one without being a knight. Look closely though and it is still a straight-line move: h1 to f1, or a1 to d1, along the first rank in both cases. What is unusual is the crossing and the fact that two pieces move at once, not the direction. Castling is written into the Laws as a king move with a rook attached.
Getting a rook to the other side
Because there are no diagonals, a rook that wants to join an attack on the far wing has to walk round. The standard route is up one file and along a rank.
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2 e g That detour has a name, the rook lift, and it turns up in most attacking games where the position is too blocked to open a file. Two tempi is the price of not having a diagonal.
What the rook gets in exchange
Straight lines are the strongest lines in the endgame. A rook on an open file reaches into the enemy position from a safe distance, a rook on the seventh rank eats pawns sideways, and two rooks on a bare back rank deliver a ladder mate that needs no help from the king.
A rook also has one advantage the knight does not: it can waste a move. Rd1-d2-d3 arrives on the same square as Rd1-d3, one tempo later, and in an endgame that is often exactly what you want. Every rook ending you ever study turns on that ability to pass the move to your opponent.