Here in chapter 25, Samuel dies, but for some reason, we're only given a short note on the matter.
So then we come to the matter of Nabal and Abigail. Nabal is sort of an odd character, mostly because of his name, which means essentially "fool" in Hebrew. It's unlikely that anyone would actually name their son "fool", but the possibility exists (especially since Nabal is a Calebite, which makes him sort of foreign) that the name means something else in another language.
David comes to be in the area near Nabal's land when it happens to be sheep-shearing time, generally a rather festive occasion. David sends a message to Nabal, asking if he could spare any provisions for his men in the midst of the celebrations. The nature of this request could be seen in two different ways, and there's a bit of evidence for either. The more generous view would be that David and his men, when they had been here before, had acted as protectors for Nabal's flock from foreign invaders or whatever sorts of things might come and attack a flock of sheep. As Nabal's men say, "They were a wall unto us both by night and day, all the while we were with them keeping the sheep."
The possibility also exists that this is a sort of mob-like protection racket. David's constant reminder that he has kept his men from bothering Nabal's men and from taking any sheep seems possibly suggestive of this, and can't be completely dismissed. The truth may be a mix of the two.
Well, Nabal responds to David's men with "Who is David? and who is the son of Jesse? there be many servants now a days that break away every man from his master." Nabal is not only denying that David has done him any good turn, but he is insulting David by suggesting that he is nothing, or less than nothing: a runaway rebellious slave to Saul. David is insulted enough that he orders his men to gear up for battle, and vows that he will kill "any that pisseth against the wall." This is indeed just about everything that SAB claims it to be, but I think there is a misinterpretation nonetheless. It's harsh, coarse language, and a promise of violence and intolerance, but it's not an indication that David has anything against a certain manner of relieving oneself, rather it's a crude way of saying, "I'm not going to leave a single man alive." After all, it's not women or young children who "pisseth against the wall" is it?
In the meantime, one of Nabal's men has gone to his wife Abigail, and told her what Nabal said, and how he fears violence will come to them because of Nabal's words. Abigail gathers up a generous amount of food, and takes it to David personally, keeping it a secret from her husband for the time being. She comes to David and bows down before him, making an eloquent plea for David to reconsider, suggesting that David should leave justice in the hand of God, using language reminiscent of David's famous fight with Goliath. Note that Abigail also ends her speech with "...but when the LORD shall have dealt well with my lord, then remember thine handmaid." I was reading that some have suggested that somehow Abigail may have been implying that she might do away with Nabal, and then everything that was Nabal's (including, especially, Abigail) would belong to David.
Indeed, the very next morning, when Nabal is probably well hung over, Abigail tells Nabal about all the food she gave away, and how David and his men nearly came to kill everyone. Nabal appears to have a stroke, and dies ten days later. Yes, the text does say "the LORD smote Nabal," but I think it's possible (but not necessarily so) that this is figurative speech beyond the fact that in some sense, God is responsible for the death of everyone.
David marries Abigail, and also some woman named Ahinoam about whom we are told just about nothing.
Saul, meanwhile, has given David's first wife Michal to a man named Phalti. Most this shuffling around of Michal is cruel to her and unlawful for Saul, but has political undertones. Michal, being the daughter of the current king, may confer some royal status on David, and Saul wishes to take that away.