If you're visiting southwestern Italy and have the chance to visit the ruins at Pompeii, I strongly suggest you do so. The place is amazing, and much larger than I expected it to be. Travel expert Rick Steves claims that you can tour Pompeii in three hours; I say that's far, far too short a time to see the place. We toured the ruins over two days; I'm guessing we spent at least 12 hours there total, and there was still stuff there that we didn't get to see.
If you're planning to see both Pompeii and Herculaneum, see Pompeii first if you can. Pompeii was much larger, having a population of about 20,000 people versus about 5,000 in Herculaneum. But, because looters first rediscovered Pompeii in the 16th and 17th centuries, it's suffered more from both erosion and plundering. The first planned excavations at Pompeii began in 1748, but it wasn't properly excavated until an Italian archaeologist named Guiseppe Fiorelli undertook the task in the mid-to-late 1800s. Archaeologists are still uncovering Pompeii's secrets today, but you'll appreciate both sites more if you see Pompeii first.
When I saw Pompeii, I was impressed by the architecture, artistry, and artisanship of the buildings. These people, in 79 AD, had indoor plumbing. It really hit me what kind of technology was lost and had to be re-learned as a result of the fall of the Roman Empire.
Some of the houses have carved phalluses on their doorways; it's my understanding that these were protective/good luck symbols. You'll see out-and-out pornographic frescoes in places like the brothel. In the brothel, you'll see various sex acts depicted; these might have been as much for illiterate customers to order services as for decoration.
The House of the Vetti and the House of Mysteries have some really wonderful frescoes and statuary still intact; you'll see what kind of influence ancient Roman artwork had on Renaissance artists such as Michaelangelo. If you want to see more of the statuary and frescoes, however, most of the better pieces have been removed and are kept in the museum at Naples.
You will see are casts of the bodies of some of the people who died at Pompeii on display in cases in the ruins. When the first archaeologists were digging through the ash, they'd come upon odd cavities in the ash. They hit upon the idea of filling the cavity with plaster and, once it had set, brushing the ash from around it. They then had a cast of the space people's corpses took up as they were incinerated by the ash flow from Vesuvius. Most of these casts have skeletons inside them; some of the cast's feet had been chipped, and I could see toe bones inside.
The casts are eerie and some terribly sad to see. You'll see children huddled near their mothers. One cast was of a small person, perhaps a child or young woman, who had slumped beside a wall, covering her face with her hands. Some find the casts morbid, but I found they provided an important reminder of the enormity of the human tragedy that happened in this place.
The ruins are often filled with roaming packs of dogs. Most of the dogs are fairly tame, but it's best to not get too close to them, since many of them are mangy or otherwise diseased. If you eat at the cafeteria (the food is decent, so there's no reason not to) expect to see a small dog or two inside begging for scraps.
And if you visit the ruins, be sure to bring your sunscreen and wear a decent pair of walking shoes; hiking boots wouldn't be out of the question. You'll do a whole lot of tromping around, and it would be very easy to turn your ankle on one of the cobblestones of the ruins' streets.