Vertical nail ridges, which run from the cuticle to the tip of the nail, are fairly common. They don't indicate serious illness and typically increase with age.
White lines and horizontal ridges, although in exceedingly rare cases can be due to arsenic poisoning, are almost certainly normal. The nails grow from the nail bed and in most people the growth across the nail bed is a little up and down, which results in a corrugated appearance. It's quite normal and not a sign of any disease process.
http://www.netdoctor.co.uk/ate/skinandhair/205541.html
Nail ridges are fairly common in older people (I hope you're not offended if you happen to be young!). There is actually a medical term, "onychorrhexis," for this condition. Splitting occurs because of dryness, which causes the nails to get hard and brittle, leading to splitting near the nail tips. This can occur spontaneously or with minor trauma. Wrapping of nails or artificial tips may actually make the problem worse by not letting the nails "breathe". Does stress make them worse? Who knows! Stress probably is related to many conditions.
Dermatologists recommend using a strong moisturizer that contains lactic acid (like Eucerin cream) twice a day, especially after a shower or when the nails are wet. It may take several weeks before you see results. If the splitting persists, you should see a dermatologist to investigate the possibility of infection.
http://healthlink.mcw.edu/article/901226269.html