Visiting the ruins at Herculaneum was quite an interesting experience. If you have the chance to see them, I highly recommend it, but you should probably see Pompeii first because the ruins at Pompeii will give you better perspective on what you'll see at Herculaneum.
To get there, you'll want to leave from Sorrento or Naples on a bus or train headed for Ercolano Scavi (Scavi di Ercolano). The ruins are open from 8:30 to 5 p.m. with final admissions at 3:30 from November through March, and they're open from 8.30 a.m. to 7.30 p.m. (final admissions at 6) from April through October. You should plan to spend at least two hours exploring the ruins, and you should enlist the help of one of the knowledgeable guides who approaches you near the entrance. They'll expect a tip, of course, but it will be worth it, because the mazelike ruins are poorly marked and are very hard to tour on your own.
As Jaz states, Herculaneum was a much smaller city -- 5,000 inhabitants versus Pompeii's 20,000. But because Herculaneum wasn't extensively excavated until the mid-1900s, the ruins are overall in much better condition because they haven't suffered as much from looting and erosion (the latter may become a problem, though, since the excavations to me seemed under-funded and proper preservation seemed lacking in places).
Some of the houses in Herculaneum have original wood beams and furniture still intact, though of course they're charcoaled. Thus, you're much better able to see how the houses were built, and other architectural details are much more apparent. And while the tilework at Pompeii is quite wonderful, in Herculaneum you'll see better tile examples where colors have stayed intact over the millennia.
As with Pompeii, the best treasures from these ruins are kept at the museum in Naples, though you will see some fine frescoes and statuary here. Probably the most impressive art you'll see here is a vibrant mosaic wall in the House of Neptune, which is toward the back of the ruins.
On the whole, visiting Herculaneum is a much quieter, more scholarly experience than seeing Pompeii. You'll see many more school groups than tourist groups, and the bookshop at the ruins is much more geared to selling actual books than tourist trinkets. So, if seeing Pompeii inspires you to read up on Roman history, you might wait to see what kind of books you can find in the Herculaneum store, because you'll see a better selection overall.
The ruins don't have a cafeteria or snack bar, but you can bring food in with you as long as you dispose of your trash properly. There are several snack shops just across the street from the ruins' entrance that will sell you sandwiches and drinks to go for a reasonable price (many of the sit-down restaurants in the area are a bit spendy).