Ostia Antica is a remarkably well preserved ancient Roman city not that far from the outskirts of modern Rome. Many people like to visit Ostia Antica while traveling in Italy, since it is an interesting spot, and some people compare it to Pompeii, another abandoned Roman city which also provides intriguing fodder for visitors. People who want to visit Ostia Antica can do so relatively easily, taking advantage of public transit or private tours to reach the site; it is a good idea to dedicate a day to wandering around Ostia, because there's a lot to see.
Modern Ostia is in the middle of a meadow, but when the area was first colonized, it was at the mouth of the Tiber River. The river has since moved seaward, due to silting and other natural changes in the environment. Archaeological evidence suggests that humans have been living at the site since around 1,500 BCE, and possibly even earlier, and according to legend, the Roman city on the site was founded in 700 BCE by Ancus Marcius, the fourth king of Rome. Many of the oldest surviving structures at Ostia Antica date from around the third century BCE, suggesting that the city may not be as old as is claimed.
The Romans originally built a city at the site as a military installation, to protect Rome from invaders. However, it soon became an important port town in its own right, with a large proportion of Rome's goods passing through Ostia at some point or another. This is one of the things which makes Ostia Antica such an interesting site to visit, as people can see what a working port town would have been like in the Roman Empire.
Ostia Antica was sacked by pirates several times in its history, and invaded by a wide assortment of others. Around the ninth century CE, the city was abandoned, and it has been left largely alone ever since, except during the Middle Ages, when some of the city was looted for building material. Despite this, many of Ostia's buildings and fixtures are intact for people to admire today, from an amphitheatre for performances to stunning mosaic floors.
Several guidebooks include sections on Ostia Antica, and there are also books dedicated specially to the city and its sights. It can help to bring a guidebook or an experienced guide along when you visit, to put the sights in context and answer questions for you as they arise.