Herculaneum, like Pompeii, was buried by the 79 A.D. eruption of Mt. Vesuvius. Unlike Pompeii, however, Herculaneum was actually more preserved than destroyed by the blast. This means the buildings are better preserved, including many that still have a second storey you can see, giving visitors a more accurate feel for what the city looked like before the eruption. The hot ash that covered Herculaneum and killed the people who couldn’t escape was deep enough that it covered two-storey buildings, sealed frescoes and mosaics on their walls, and even encased food supplies in household kitchens.
Much of Pompeii is off-limits to visitors, with doorways blocked by ropes you can’t cross and buildings you can’t enter. Part of this is due to the instability of the structures (collapses of walls or entire buildings are not uncommon, sadly). At Herculaneum, there are some areas that are roped off, but there are also many buildings you can walk right into. The experience is eerie, as you expect the homeowner to come into the room at any moment, admonishing you for entering without knocking on the door.
As mentioned, the excavated site at Herculaneum is far smaller than that of Pompeii, and even today the modern city of Ercolano sits above more of the ancient city that hasn’t been unearthed. Pompeii is also far more visited, which means you’re less likely to encounter the same kinds of big crowds at Herculaneum. It sits slightly closer to Naples, so you can plan a day trip from Naples that includes Pompeii in the morning, a break for lunch, and then Herculaneum in the afternoon on your way back.