What merits a win/loss?

Question
I've been a casual baseball fan for awhile and there's still something that seems like a bit of a gray area to me: How they determine which pitcher earns the win/loss in a game. I used to think it was simply based on innings pitched, but that's clearly not the case. Example: @ a Tigers game back in May, Nate Robertson pitched 7 1/3 scoreless innings, then Fernando Rodney came in, pitched 2/3 innings, and got the W. What does MLB go by when determining this? Thanks.

Answer
Joel,

There are some exceptions, but in most cases the pitcher that is in the game when the team takes a lead that it never relinquishes gets the win.

One example:

Joe Blow pitches six scoreless innings against the Muffins.  But his team also doesn't score.  In the seventh inning Bob comes in a pitches a scoreless inning.  Then his team scores a run and the game ends 1-0.  Bob will get the win, even if he doesn't pitch any more.  

In that example though, if Bob's team then gives up a run after the first run is scored tying the game, then Bob won't get the win unless he is in the game when the go ahead run is scored.

If a starting pitcher does not pitch five complete innings, then they can't get the win no matter what the score.  They are required to pitch five to get the win.

Please ask more questions if you need any clarification.

Hope this helps!

Brian