This is a very good question and if you do not you better start. Normal nails should be light pink, intact, and semi transparent; if you notice some white dots or lines, it could be an infection by fungus or bacteria; it is advisable to keep your nails dry and clean to protect them.
Furthermore, any illness in your body can cause changes in your nail appearance; that is why watching out for these changes is important. The whole nails could look white or cloudy which indicates renal failure, heart disease, rheumatoid arthritis or diabetes. Liver cirrhosis and HIV can make the nails look grainy with a red or pink strip at the top due to a decreased blood supply in the nail bed.
Sometimes, horizontal white lines appear on the nails especially the second, third and fourth fingers, which is a result of the abnormal blood flood in the nail bed associated with liver disease, malnutrition and chemotherapy. When the bottom half of the nail is white and the top half is dark, this could mean HIV or a renal disease.
Dark brown or red vertical lines appearing on the nail bed can be caused by a lot of things including but not limited to oral contraceptives, rheumatoid arthritis, peptic ulcer disease and pregnancy.
Vertical brown streaks are common among dark skin people but they may indicate melanoma; so it is better checked by a doctor.
Apart from the small lines appearing, there could be changes in the nail texture. A deep groove and a disruption in the nail growth could mean peripheral artery disease, psoriasis or trauma.
Clubbing means the nails become harder, thicker and shaped like a bulb and it is usually associated with heart, lung and liver diseases and may indicate cancer. All in all, any nail change can signify medical problems and should not be underestimated.