You can learn a lot from Wikipedia. Here’s what they have to say.
The Camino de Santiago or “The Way of St. James was one of the most important Christian pilgrimages during the Middle Ages, together with those to Rome and Jerusalem, and a pilgrimage route on which a plenary indulgence could be earned; other major pilgrimage routes include the Via Francigena to Rome and the pilgrimage to Jerusalem.
Legend holds that St. James’s remains were carried by boat from Jerusalem to northern Spain, where he was buried in what is now the city of Santiago de Compostela. (The name Santiago is the local Galician evolution of Vulgar Latin Sancti Iacobi, “Saint James”.)
The Way can take one of dozens of pilgrimage routes to Santiago de Compostela. Traditionally, as with most pilgrimages, the Way of Saint James began at one’s home and ended at the pilgrimage site. However, a few of the routes are considered main ones. During the Middle Ages, the route was highly travelled. However, the Black Death, the Protestant Reformation, and political unrest in 16th century Europe led to its decline. By the 1980s, only a few pilgrims per year arrived in Santiago. In October 1987, the route was declared the first European Cultural Route by the Council of Europe; it was also named one of UNESCO’s World Heritage Sites. Since the 1980s the route has attracted a growing number of modern-day international pilgrims.
Whenever St. James’s Day (25 July) falls on a Sunday, the cathedral declares a Holy or Jubilee Year. Depending on leap years, Holy Years occur in 5-, 6-, and 11-year intervals. The most recent were 1982, 1993, 1999, 2004, and 2010. The next will be 2021, 2027, and 2032.
Santiago de Compostela is a World Heritage City, pilgrimage destination, cultural capital and example of historical, urban and environmental regeneration, which attracts thousands of visitors..”
There are several “ways”. There is the France way from southern France, there is the Spanish way which can be joined at many different locations, if desired. There is the English way from the coast of Northern Spain and there is Portuguese way leading from Oporto, or parts south as far as Lisbon.
There are different types of organised ways including group and self guided walking and cycling options.