I quite don't understand this whole thing. Jesus and his disciples are hanging around the city all the time. Why can't the Pharisees arrest him at their leisure? Why do they need Judas to tell the soldiers which one Jesus is?
It's something you probably don't think about much, but we have reason to believe that Jesus really wasn't all that distinctive in his appearance.
58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
59 Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
28 And all they in the synagogue, when they heard these things, were filled with wrath,
29 And rose up, and thrust him out of the city, and led him unto the brow of the hill whereon their city was built, that they might cast him down headlong.
30 But he passing through the midst of them went his way
This is the act of someone who is very ordinary-looking and able to blend in with a crowd at will. See also Isaiah 53:2. The context makes it clear that he's prophesying about the Messiah here, and he clearly states that he does not look like what you'd expect of a powerful, charismatic leader:
...he hath no form nor comeliness; and when we shall see him, there is no beauty that we should desire him.
As Dancek pointed out, Jesus reminded them that they could have come for him any time he was in the temple teaching. It's not hard to find him then: look for the one who's the center of attention! But looking for him at night, when you don't really know what he looks like and even if you did, that description might match any of thousands of Jews? (And this was Passover season, so there were a lot of extra Jews in Jerusalem at the moment.) The only way to make sure you've got the right guy in a situation like that is to get someone who personally knows your target to identify him. And so that's exactly what they did.
Jesus makes the same point when he is being arrested. He certainly could have been seized anytime without Judas's help.
Mark 14:48-49 (ESV)
48 And Jesus said to them, "Have you come out as against a robber, with swords and clubs to capture me? 49 Day after day I was with you in the temple teaching, and you did not seize me. But let the Scriptures be fulfilled."
The Scripture had to be fulfilled, and that's why the arrest had to happen this way. I think this might refer to Isaiah's prophecy on the Messiah being counted a criminal.
Isaiah 53:12 (ESV)
Therefore I will divide him a portion with the many,
and he shall divide the spoil with the strong,
because he poured out his soul to death
and was numbered with the transgressors;
yet he bore the sin of many,
and makes intercession for the transgressors.
You have to remember the time period.
Back then they didn't have photographs. In fact, paper and horseshoes were still new inventions. The soldiers that were sent to capture him weren't his disciples, so they wouldn't know his face, only his reputation.
As to how he betrayed him:
Upon coming up to Jesus with the soldiers he kissed him.
And forthwith he came to Jesus, and said, Hail, master; and kissed him.
Luke 22:2-6 KJV
And the chief priests and scribes sought how they might kill him; for they feared the people. Then entered Satan into Judas surnamed Iscariot, being of the number of the twelve.
And he went his way, and communed with the chief priests and captains, how he might betray him unto them. And they were glad, and covenanted to give him money. And he promised, and sought opportunity to betray him unto them in the absence of the multitude.
Jesus meet with the disciples in reservated separated places, "in the absence of the multitude" to explain better doctrines, to pray, to rest, etc. The chief priests and scribes expected that opportunity to arrest Jesus and Judas help them...
The pharisees did not want to arrest him in public for the fear of the people. During day time Jesus was always surrounded by the multitude.
So it was necessary that he should be arrested during night. But He might not be that distinctive in the night as the disciples also might have similar dress as he wore. Besides, the pharisees could not be sure about where Jesus would be at a particular night.
The Roman soldiers were sent to arrest him. They might not be very familiar with Jesus, so they did not want to make a mistake by arresting the wrong man...
First of all, the appearance of Jesus was ordinary. According to Isaiah 53,He has no form or comeliness; and when we see Him, there is no beauty that we should desire Him. That means that Jesus appears as anyone of His disciples. Only Judas who was with him all the time could acknowledge Him. Second, the betrayal the Bible is talking about is more of the heart. Jusah was wicked in his heart and never honored Jesus as it's due. He wanted to "intentionally" kill him. Lastly, the Pharisees needed to make sure they're not arresting the wrong person. They intended to arrest Him long time ago, but they were afraid of the crowd because they take Him as a prophet.