Essentially the Inca Trail is a mountainous jungle hike leading to the sacred Inca city of Machu Picchu. The 45km trek is usually covered in 4 days, arriving at Machu Picchu at daybreak on the final day before returning to Cusco by train in the afternoon.
The trek is rated moderate and any reasonably fit person should be able to cover the route. It is fairly challenging nevertheless, and altitudes of 4200m are reached, so ensuring that you are well acclimatized is important. If arriving from sea level, plan to spend at least 2 full days in Cusco (3 days is better) prior to commencing the trek. This should allow plenty of time for acclimatization and give you sufficient opportunity to visit the city of Cusco and nearby Inca ruins at Sacsayhuaman, Q'enko, Pucapucara and Tambomachay, as well as spending a day or two exploring the Sacred Valley of the Incas visiting the tradition market town of Pisac and the fascinating Inca fortress at Ollantaytambo.