I'm sure everyone knows what happens next. Cain gets pissed off, and totally overreacts by killing his brother. When God finds out (it takes a little while for Him to figure it out, as He apparently hadn't gotten the whole omniscience thing down yet), He forces Cain to be a wanderer, but also gives him a mark that signifies he would be avenged sevenfold if anyone killed him. Not exactly the punishment He prescribes for murderers in Moses' time, but I guess God isn't too keen on vigilante justice. Even the wandering thing doesn't seem to have been too bad for Cain, since he's soon gotten married, had a son, and founded a city. I'm surprised he was able to find anyone to marry, but the traditional belief was that he married his sister. Yeah, only four chapters into this holy book, and we're already getting incest. But really, a lot of this doesn't add up. If Cain is one of three people on Earth, why does he think someone will kill him? Does he think his parents would do that? How could he be avenged sevenfold when there aren't seven people alive yet? And why would he need a city for only three people? All this suggests to me that the Cain story wasn't originally intended to be so closely linked to that of Adam and Eve. In fact, they're only in the first two verses of this chapter, after which they disappear from the story until Verse 25. You'd think they'd have more of a reaction to their one son killing the other, wouldn't you? So is it possible that Cain and Abel were originally supposed to have lived some time later, after there were more people around who would have wanted to kill Cain and/or live in his city? For that matter, maybe it comes from another tradition in which humanity started out as a population rather than an individual, but that's just speculation on my part. The story can also be regarded as symbolism, not just of man's inhumanity to man, but also of the rise of agriculture. It supplanted the nomadic herding system and led to the rise of cities, after all.
Regardless of the meaning, the chapter goes on to list Cain's descendants, culminating in the fathers of three different trades: the cattle-herder Jabal, the musician Jubal, and the metal-worker Tubalcain (an important figure in Freemasonry, from what I've heard). Then we get back to Adam and Eve and their third son Seth, and none of Cain's descendants are mentioned again.