One of the clear differences between the texts is that Xenophon makes more of an effort to put the speech in a context. He is very concerned, for instance, with the fact that Socrates comes across as high-handed in his defense speech (indeed, he comes across as more high-handed in Xenophon's than Plato's). With Plato we just get the speech. With Xenophon, we get the lead-up to the trial, in a section that is very, very close to parts of the last section of the Memorabilia, and then the speech, and then we get some events in the aftermath of the speech.
You can read Xenophon's Apology online in English at the Perseus Project.
Xenophon's source. He is mentioned in Plato's Phaedo and is a character in the Cratylus.
One of the accusers; Socrates will dialogue with him during the speech, just as in Plato.
Apollodorus of Phaleron
Both Plato and Xenophon depict Apollodorus as being very emotional; Xenophon describes him as unintelligent. He happens to be the narrator-character of Plato's Symposium, and he will be with Socrates on his last day.
One of the accusers; he does not speak here, although Socrates makes some remarks as he walks by. He is, of course, a character in Plato's Meno, where he has an irrational hatred of sophists despite not knowing anything about them.
In addition, there are a great many jurors and spectators
The Plot and The Thought
Xenophon opens by reflecting on the fact that others having written on Socrates' trial have touched on his high-handed tone, and says that what they've failed to convey is that Socrates was already willing to go to his death. He then gives a version of the discussion with Hermogenes that is also found at the end of the Memorabilia.
With this preface, he gives a summary version of Socrates' defense speech. First, Socrates addresses the charge that he does not recognize the gods of the state, pointing out that everyone has seen him performing sacrifices on the appointed days. If it's the divine voice that's the problem, however, is it really so much of a problem? Listening to voices with divine force is found throughout Greek culture, from people taking bird-calls or thunder to be omens, all the way to the Pythoness herself at the Oracle at Delphi. Everyone recognizes that the god knows the future and can communicate it however he wills. But his divine voice is as divine as these things, and more, and it has never been wrong.
There's an uproar among the jury at this point, but Socrates charges on with the story of Chaerephon and the Oracle: Chaerephon went to the Oracle and the Oracle said that Socrates was the most free, most upright, and most prudent of all people. Which, of course, causes an even greater uproar. To which Socrates replies that it's not so amazing; the Oracle once said of Lycurgus that it could not tell whether he was a god or a man, but the Oracle never suggested that Socrates was a god, only that he was better than all other men.
Moreover, they should investigate the Oracle's claim. Can they find anyone more free from enslavement to bodily desires than Socrates? Does he not refuse fees? Has he not divested himself of unnecessary things like an upright person? And has he not been investigating and learning every good thing he could since he learned how to speak? Did not many Athenians who were looking to become virtuous associate with him? And so on, and so forth.
But what about the charge of corrupting the young? Socrates demands of Meletus to name one person who has become a drunkard, or an atheist, or a libertine, because of Socrates. Meletus replies that there have been plenty of young people who have learned from Socrates not to listen to their parents.
Socrates replies that everyone recognizes that there are occasions where you should listen to other people and not your parents -- for instance, people trying to get well should listen to doctors rather than their parents. And Athenians elect people whom they think have knowledge for important positions like strategos. So it is strange that Meletus is prosecuting him because other Athenians think he knows something relevant to education.
Xenophon breaks off his summary here, saying that he is not interested in giving everything that was said, but simply showing that Socrates considered it important to answer the charges, but did not think it good to beg for his life. This is seen in the fact that he refused to propose a counter-penalty, or to let his friends do it; and by the fact that when his friends wanted him to go away secretly, he refused to go. Xenophon skips to Socrates at the end of the trial. Socrates in his last speech is defiant, saying that the fact that they have voted him guilty leads to him lowering his own opinion of himself not one bit, because they have not actually shown that he is guilty of impiety or corrupting the youth.
Nor should he be ashamed by the injustice of the condemnation, since the shame belongs to those who voted for it. He takes comfort in the example of Palamedes, who was unjustly killed by Odysseus, and yet is given greater eulogies, and ends by saying he has no doubt that those in the future will testify that he did no wrong but instead "benefited those I conversed with by freely teaching them any good thing I could" (section 26).
At this he was led away, but when he met any of his friends weeping for him, he replied that they should remember human nature condemned him to death since the day he was born. This brings us to one of my favorite Socratic passages:
One of those present was Apollodorus, who was a great devotee of Socrates, but was not particularly bright. He said, 'But the most difficult thing for me to bear, Socrates, is that I see you being unjustly put to death.' Socrates (as the story goes) stroked Apollodorus' head and replied with a smile, 'You're a good friend, Apollodorus, but would you rather see me put to death justly or unjustly?' (section 28)
Socrates also sees Anytus walking by, and comments that he walks as if he has done something excellent, when he is actually putting Socrates to death because he heard that Socrates made the comment, when Anytus was trying to get political office, that he should not be educating his son in a tannery. But he is really the worse off, because the victor of the contest is the one whose achievements will be the more excellent. Then he prophesies that Anytus' son will end badly because of the education his father gave him. And Xenophon says that Socrates turned out to be right.
Thus, says Xenophon, Socrates more or less forced the jurors to condemn him, and by doing so he died before senility and in an easy way, with the greatest kind of fortitude, never flinching from his death. And Xenophon ends with a famous passage of his own:
When I consider how wise the man was, and how high-minded, I am bound to remember him; and when I remember him, I am bound to admire him. If anyone in his search for virtue has encountered a more helpful person than Socrates, then he deserves, in my opinion, to be called the most fortunate of men. (section 34)
Quotations are from Xenophon, Conversations with Socrates, Treddenick and Waterfield, trs. & eds. Penguin (New York: 1990).