This from wiki....
, a coconut is a simple dry fruit
known as a fibrous drupe
(a true nut
). The husk (mesocarp
) is composed of fibres
and there is an inner "stone" (the endocarp
). This hard endocarp (the outside of the coconut as sold in the shops of non-tropical countries) has three germination pores
that are clearly visible on the outside surface once the husk is removed. It is through one of these that the radicle
emerges when the embryo
germinates. Adhering to the inside wall of the endocarp is the testa
, with a thick albuminous endosperm
(the coconut "meat"), the white and fleshy edible part of the seed. The endosperm surrounds a hollow interior space, filled with air and often a liquid referred to as coconut water
, not to be confused with coconut milk
. Coconut milk
is made by grating the endocarp
and mixing it with (warm) water. This produces a thick, white liquid called coconut milk
that is used in much Asian cooking, for example, in curries. Coconut water
from the unripe coconut, on the other hand, is drunk fresh as a refreshing drink.
When viewed on end, the endocarp and germination pores gives to the fruit the appearance of a coco
(also CŰca), a Portuguese
word for a scary witch from the Portuguese
folklore, that used to be represented as a carved vegetable lantern, hence the name of the fruit 
. The specific name nucifera
is Latin for nut bearing
When the coconut is still green, the endosperm inside is thin and tender, a favourite snack. But the main reason to pick the nut at that stage is to drink its water; a big nut contains up to one litre of refreshing drink. When the nut has ripened and the outer husk has turned brown, a couple of months later, it will fall from the palm of its own accord. At that time the endosperm has thickened and hardened, while the coconut water has become somewhat bitter.
Coconut flower. Location: Taliparamba
To open a coconut, remove the outer husk (if not purchased already removed) and pierce two of the three eyes
of the fruit (one for the juice to come out of, one to enable air to go in); drain the juice
from the fruit. Since coconuts have a naturally-forming fracture point, they can be opened by taking a heavy knife, such as a meat cleaver, and striking the coconut with the flat edge of the knife. Or you can use a flat-bladed screwdriver and a hammer (which is easier, and may be safer than using a cleaver). After inserting the screwdriver slightly, twist it to crack the shell. The coconut should then be turned, and this process repeated until there is a contiguous crack in the shell around the entire fruit. Afterwards, the fruit can be separated at this fracture point.
More here: http://en.wikipedia.org/wiki/Coconut