Nails grow all the time, but their rate of growth slows down with age and poor circulation. Fingernails generally grow faster than toenails at a rate of ~3mm per month. It takes an average of 6 months for a nail to grow from the matrix to the free edge. Toenails grow about 1 mm per month and take 12-18 months to be completely replaced. Interestingly enough, the nail plates grow at about the same speed continents move!

Nail Structure

The structure we know of as the nail is divided into six specific parts - the matrix, the nail bed, the nail plate, the eponychium, lateral folds and the hyponychium. Each of these structures has a specific function, and if disrupted can result in an abnormal appearing fingernail.

The Matrix

This is the most important part of the natural nail plate as it is solely responsible for incubating new nail plate cells. Any damage to the matrix will result in permanent damage to the natural nail plate. The matrix lies beneath the proximal fold (at the base of the nail plate) and is several millimeters in length. As new nail plate cells are incubated, they emerge from the matrix round and white. This in turn pushes the old nail plate cells upward and outward slowly compressing older cells before them. After several weeks, the round, white nail plate cells become compressed, flat and translucent which is how you can see the pink colouration from the millions of capillaries in the nail bed. An interesting and important point to note is that the proximal (closest to you) end of your matrix produces the top layer of the natural nail plate while the distal (furthest point from you) end of the matrix produces the bottom portion of the natural nail plate. The distal end of the matrix is often seen as a white moon called the lunula. The lunula is the distal end of the matrix and the round white cells are the reason for the lunula being white in colouration.

Nail Bed

The nail bed is comprised of two types of tissues: The epidermis and the dermis. The dermis is living tissue and contains capillaries and glands (i.e. sebaceous and suderiferous - oil and sweat glands respectively). The dermis is fundamentally fixed to the bone and is stationary. An interesting and important feature is that this dermal tissue contains many 'grooves' that run the length of the nail bed.
The Epidermal tissue is a special type of epidermis called the bed epithelium. The bed epithelium non living tissue that is produced at the distal end of the matrix. While this skin is produced, it becomes partially 'fused' with newly formed nail plate. This bed epithelium then grows out with the natural nail plate, one side stuck to the bottom of the natural nail plate and the other side stuck in the 'grooves' of the dermis (the true nail bed).

Nail Plate

The nail plate is the actual fingernail, made of semi-translucent keratin. The pink appearance of the nail comes from the capillaries beneath the nail.

The Eponychium

The eponychium is most often mistakenly called the cuticle (which it is not!). Less frequently, it is called pterygium (which is a serious nail disorder!). The eponychium is the end of the proximal fold that appears to end at the base of the natural nail. The eponychium does not end there, it in fact folds back upon itself, hence the reason it is refered to as a fold.
Where it folds back upon itself, it sheds an epidermal layer of skin onto newly formed natural nail plate. This epidermal layer of non living skin then 'rides out' attached to the surface of the natural nail plate. This non living (almost invisible) layer of skin is the cuticle that is removed during manicuring. This relationship between the cuticle and the eponychium creates a perfect seal that keeps pathogens away!
Though the cuticle is removed during nail services, it can not be removed with nippers, only with a pusher and professional cuticle remover products. Cutting or clipping of living tissue (in this case the eponychium) should always be avoided unless you want to risk a serious nail infection like paronychia.
It is important to always remember to not push back the eponychium aggresively. Doing so tears the eponychium away from the cuticle and increases the risk of an infection.

The lateral folds

The lateral folds are a natural extension of the proximal fold and the eponychium and help guide the nail plate out to the free edge.

The Hyponychium

The hyponychium is the area between the nail plate and the fingertip. It is the junction between the free edge of the nail and the skin of the fingertip.