Oswiecim is a mid-sized Polish town in the Silesian region, and it's primarily visited for one reason: it is the location of Auschwitz-Birkenau, the Nazi German concentration and extermination camp where more than a million people — the vast majority of them Jews — were murdered during World War II. The Auschwitz-Birkenau State Museum is one of the most visited sites in Poland and a UNESCO World Heritage Site.
The camp complex is divided into two main parts: Auschwitz I, the original camp with its infamous "Arbeit Macht Frei" gate and the central museum buildings; and Auschwitz II-Birkenau, a few kilometers away, which is the larger extermination camp with the rail spur, ruins of the crematoria, and vast field of barracks. Both are essential to visit together, and both are sobering in very different ways. Guided tours are strongly recommended; visiting without context leaves people poorly equipped to understand what they're seeing.
Oswiecim before the war was a different place: roughly half the town's population was Jewish, with a community dating back several centuries. The Oswiecim Synagogue and Jewish heritage sites offer a glimpse of what existed before the occupation. Most visitors arrive from Krakow (about 70 km east) by train or bus and return the same day. There is accommodation in town if you want to visit early in the morning before the crowds arrive, which is worth considering in summer.