What causes ridges on nails?

There are numerous causes for ridging. But, you didn't say which way the ridges are going, because there are different reasons for those ridges which are horizontal (across the nail plate) from those which are vertical (the direction that the nail grows). Having said that, the most usual causes for vertical ridging tend to be either trauma to the nail unit, or aging, as well as other skin conditions, such as psoriasis. The most common cause for horizontal ridging is a momentary stoppage of nail growth, as is seen when you get sick with a fever or some other stressful situation to your body. (this is called a Beau's line).