Why it's called the "W"
The circuit gets its name from the shape it traces on the map: three side valleys walked out and back from the park's main trail, linking the three viewpoints that made Torres del Paine famous. The full route covers 60 to 80 km depending on the starting point, spread over four or five days with nights at refugios or campsites inside the park, all requiring a mandatory, limited-capacity reservation.
The three arms of the W
The first valley climbs to the Base Torres viewpoint, where three granite spires rise above a glacial lagoon; it's the steepest climb on the circuit, especially the final stretch over boulders. The central valley leads to the French Valley, with views of hanging glaciers that break off in avalanches audible from the trail on clear days. The third arm ends at Grey Glacier, where the circuit touches the shore of the lake of the same name and its calved icebergs, with the option of a catamaran ride to the glacier's face for those not continuing onto the full circuit (the "O").
How to plan it
The park's refugios and campsites are booked separately depending on each sector's operator, and in high season (December to February) they sell out months in advance; it's worth planning the full itinerary before buying the flight. Patagonian wind, which can exceed 100 km/h in exposed areas, is what changes the experience most day to day, so layered clothing and a trekking pole make more difference than fitness level.