The well preserved city has been extensively, though not completely, excavated from the ash from eruption of Mt. Vesuvius in 79AD. Almost 2,000 years later and you can get a really good sense of the lives of the residents.
Pro Tip: Download an audio guide before you head out or hire one. There is a lot to see and you don't know what you're looking at when you walk through on your own.

The blocks in the road allowed citizens to walk through wet/muddy streets and keep their togas clean.
The site is still being uncovered.
A Roman bath house. This is the steam room.
Fast food. Pots of food would be put in the holes on this street side stand.
The ash has preserved mosaic floor like this in ways other sites could only hope for.