Why Do My Nails Have Ridges?

Do the ridges run across the nail, or from cuticle to tip?
If they run across the nail they are called "Beau's lines".  These ridges are due to irregularities in the nails growth.  This may be from trauma - such as hitting your finger with a hammer, or because you have had a serious illness or shock within the last few months, or because there has been some interruption in the blood supply to that finger.  However, some peoples nails grow irregularly for no discernible reason.
If the ridges run from cuticle to tip this again may just be a personal peculiarity.  Or it can indicate some illness.  These could include psoriasis, and various autoimmune diseases.
Ridging can also be caused by nail biting, persistant fiddling with the nails (onychotillomania) or over zealous manicure.