Camino de Santiago or Camino Frances is an ancient pilgrimage trail, in use for over 1000 years. There is a belief that St. James the disciple's bones were found in northern Spain, and are interred in the cathedral in Santiago de Compostela, Spain. People would travel there to get forgiveness for their sins. The tradition goes on today, and in Spain there is a system of refugios - similar to youth hostels, that provide shelter for pilgrims for a minimal fee or contribution. See Map of route in Spain. For a larger area map showing all routes from Europe, see the map from our Pilgrim Passport, that we got in Roncesvalles. If starting from Le Puy in France, go to our third Camino page.
Spain is just the end point of the trail. There are trails from all parts of Europe, that merge as they go into Spain. The Spanish part of the trail is about 450 miles. The pilgrims today do it for various motives - religious, spiritual, other personal reasons. Do read Nancy Frey's book for a good sense of who the modern day pilgrims are and why they do it.
. Very few do it for frivolous reasons. You just carry a sleeping bag and personal gear, as shelter is available and you can buy food in the villages along the way. Even with this, we as experienced backpackers, found it to be a demanding trip. It is easy to get caught up in a refugio mentality - eager to get to the next one in time to get a bed. You can only stay one night, and usually have to be out by 8am the next day. There are a number of good web sites with more info, and for English speakers, a few classic books
http://www.backpack45.com/camino2.html