When a pilgrim — a peregrino — finishes the Camino, they can receive a certificate from the Cathedral, provided that the peregrino had walked (or rode a horse) at least 100 kilometers (around 62 miles), or had ridden a bike at least 200 kilometers. In order to prove the distance traveled the peregrino carries a credencial, also called a "Pilgrim's Passport". It is also used to "prove" to the alburges that one is a peregrino and not just some tourist looking for a cheap bed for the night.
The credencial is a small booklet, a little like a real passport. Like a real passport, a peregrino gets the Credencial stamped at various points along the route. Rather than getting it stamped when entering a new country, however, the peregrino seeks out stamps from alburges, churches, bars, and cafes. It is recommended that it gets stamped at least once a day when one is more than 100 kilometers away from Santiago, and twice a day once one is closer. Many peregrinos like to collect stamps — or sellos — more frequently than that.
A credencial can be picked up at the Pilgrim's Office when one starts their camino, or it can be ordered in advance from the national pilgrim's association. Each association prints a different credencial. Being American, my national association is American Pilgrims on the Camino. If you ask them, they'll send you a credencial for free.
Here's what I got from them:
If my math is right, it has spaces for 56 stamps.