Thirteen squares from d4, seven from a1. The bishop is the piece whose value swings most with where you park it, and the swing is not subtle: in a corner it is worth about half of what it is worth in the centre.
h 1 Any number of squares along a diagonal, forwards or backwards, and it captures by landing on the square. It cannot jump, so a bishop on c1 with pawns still on b2 and d2 has precisely zero moves, which is why getting the bishops out is half of what the opening is about.
One colour, for the whole game
Look at the highlighted squares again. Every one is dark. That is not a coincidence about d4: a diagonal step always changes the file by one and the rank by one, and that combination always lands on the same colour. So a bishop starting on f1, a light square, is a light-squared bishop until it is captured or the game ends. It can never change colour, not by moving, not by capturing.
That single fact decides more endgames than most beginners expect. Here is a light-squared bishop that is completely powerless.
8 
g Half the board is invisible to it. That is why two bishops working together are worth so much: between them they cover every square on the board, and the pair is generally reckoned to be worth about half a pawn more than the sum of its parts. There is a page on the bishop pair and on good and bad bishops, which is the name for the difference between a bishop in front of its own pawns and one stuck behind them.
Bishop or knight?
They are both worth about three pawns, so trading one for the other is fair on paper. In practice it depends on the position: open board with pawns on both wings, take the bishop; locked pawn chains with a hole to sit in, take the knight. Beginners tend to undervalue the bishop because it does nothing for the first six moves and then suddenly rakes a diagonal from b2 to h8.
The tactic to learn first is the pin. Play 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 and the bishop on g5 looks straight through the knight on f6 to the black queen on d8. The knight cannot move without dropping the queen, and it has not been touched. That is a bishop doing its best work from four squares away.