What's the difference between "deserve" and "need"?

In the movie The Dark Knight, at the end, there is one memorable line "Not the hero we deserved but the hero we needed".

So, what is the difference between "deserved" and "needed"?

I don't agree with the interpretation of this line in The Dark Knight in the other answers, so here's my answer.

First, definitions (as per Random House Dictionary.) Deserve means "to be worthy of, qualified for, or have a claim to reward, punishment, recompense, etc.". Need means "to require" and usually refers to things you lack that are useful or necessary.

So there is ambiguity in the word deserve. You could deserve a reward or a punishment. There is an English phrase "you get what you deserve" which usually refers to a punishment, rather than reward.

Therefore the phrase in the movie "Not the hero we deserved but the hero we needed" is (delightfully, to me) ambiguous, and actually means two things at once! Does Gotham deserve punishment or reward?

I choose to interpret it this way: because it is The Dark Knight, it is a flawed hero, not a perfect hero, not a shining paragon, so Gotham deserves better… but… Gotham is flawed, so it needs a "Dark Knight" to match its own darkness. Conversely, because of Gotham's flaws and sins, it deserves a hero even worse than The Dark Knight — but it doesn't get that. It gets what it needs, a hero who overcomes these flaws.

Difference between deserve and need :

1.You Need:

When you require something to continue/ complete any action

a) The Citizens of Gotham City needed Batman to save them from the criminals -- Here we are looking at a situation where Gotham City had a lot of criminals and the citizens had to get help

b) Brad need to win the race , so he decided to work hard

2.You Deserve :

When you have earned the right to something based on your actions

Example:

a) The Citizens of Gotham City were not grateful at all!! They did not deserve a hero like Batman. b) Brad had been working hard for the race. He deserved the 1st Place

The other answers have forgotten that the quote Morgan mentions actually applies to Harvey Dent not Batman. But I only worked out the meaning of the quote after I read the comment about being flawed. Let me explain...

I think at Harvey's fake funeral Gordon says that Harvey "is not the hero we deserved but the hero we needed" (perhaps someone else can correct or confirm the timing of this).

Gotham is flawed. It needs a good example or sign of hope that the city can rise above its flaws. In the first film Bruce mentiones that the city needs strong examples to shake it out of apathy. Batman shook it up, but now the city needs a white knight, a good example. So the city can start to rise out of the darkness.

Of course The Joker has been trying to prove that it can't. That it's no better than him. Of course that leads to Harvey's down fall and transformation into Two Face. (Bringing Harvey down to The Joker's, Batman's, and the city's level — flawed.)

Then at the end of the film Gordon's son asks why Batman is running. Gordon answers "Because he's the hero Gotham deserves, but not the one it needs right now."

As mentioned Batman is a flawed hero. Using but also fighting his darkness. Just like Gotham. So he's the hero they deserved, a Dark Knight. But not the one they needed, Harvey Dent the White Knight.

The hero in the movie did many good things for the city, so the city needed him. However the people didn't really appreciate him, so they did not deserve him.