Pompeii was a prosperous Roman city of 20,000 people when Mount Vesuvius erupted on the afternoon of 24 October 79 AD. Pyroclastic flows buried the city under 4–6 metres of volcanic ash in a single day. The speed and the depth preserved everything — streets, frescoes, graffiti, furniture, food, and the void-shapes of the people who didn't escape.
Serious excavation began in 1748. Today 66 of the estimated 110 hectares are open, and the archaeological park is still digging — the most recent major finds (a thermopolium with preserved food, a chariot) were made in 2020–2021. Expect some areas to be closed for ongoing excavation on your visit; expect others to have just opened.
The headline sights — Forum, House of the Faun, Villa of the Mysteries, amphitheatre, Lupanare — are spread across 3 km of streets. Most visitors walk 8–10 km during a day here. Allow at least 4 hours; a full day if you want to actually see it.