Not just for looks
Besides their ornamental role, nails serve several important less appreciated functions. They act as weapons of offense or defense, enable you to scratch that annoying itch, protect the delicate fingertips from injury ( imagine slamming the car door on your fingers without having your fingernails to protect them) enhances fingertips sensitivity ( you can discern whether you have two or three pages between your fingers ) and facilitates fine tactile movements such as picking up tiny objects like pins or buttoning a shirt or blouse.

The Matrix
Fingernails, just as hair is, are just an extention of the skin. They are made out of dead cells , from a protein called keratin. Contrary to popular belief, nails do not breathe.
The matrix, is the one who receives the blood flow, it is the one responsible of creating these dead cells called nails.
Contrary to popular belief, calcium does not stimulate nail growth, it is the matrix that produces the nails directly. That is why we need to protect it from being exposed to hits and careless treatment with hammers or work related tools, so to avoid incorrect nail growth.

Growth
Nails grow constantly: approximatly 0.1 mm per day and an average of 3 to 5 mm. per month.
Nail growth rate: Is affected by a number of activities and environmental conditions. For example, during the summer, nails grow faster than in winter.
Nails grow faster on the longest finger and on the predominant hand.
Fever and serious illness slow the growth rate, while pregnancy enhances it.
Minor trauma such as nail biting also stimulates nail growth, while a sedentary lifestyle and a poor nutrition slow it.
Nails grow at a faster rate in men and younger people.
Toenails grow at one half to one third the rate of fingernails.

Why do nails grow at different rates
Just as the skin is stimulated by friction to produce a callus, injury to nails can provoke healing by stimulating the matrix to regenerate the nail at a faster rate.
During the day, hands are more active, which results in an increase in blood flow and at the same time, nails and fingers are subjected to repeated mild traumas that causes nails to grow faster.
Longer fingers get exposed to more trauma, those fingers tend to grow faster than their counterparts.
During the summertime we tend to be more active, our blood circulation increases, we are warmer, causing our nails to grow faster.
Toenails grow slower than fingernails because toes are less mobile than fingers and blood circulation is less effective in the feet.
Pregnant women however, have an elevated metabolic rate which increases nail growth during those months.
Age is a very important factor because of the general body condition, the metabolism, blood circulation, activity and health are generally at their highest level in younger persons, thus making nail growth optimal compared to the later stage of life.