Many animals have differences between the male and female sexes in size and appearance, a phenomenon called sexual dimorphism. Sexual dimorphisms are often associated with sexual selection – the competition between individuals of one sex to mate with the opposite sex.[26] Antlers in male deer, for example, are used in combat between males to win reproductive access to female deer. In many cases the male of a species is larger in size; in mammals species with high sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males.
Other animals, including most insects and many fish, have larger females. This may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm—larger females are able to produce more eggs.[27] Occasionally this dimorphism is extreme, with males reduced to living as parasites dependent on the female.
In birds, males often have a more colourful appearance and may have features (like the long tail of male peacocks) that would seem to put the organism at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle.[28] This hypothesis says that, by demonstrating he can survive with such handicaps, the male is advertising his genetic fitness to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps.
Sex differences in humans include, generally, a larger size and more body hair in men; women have breasts, wider hips, and a higher body fat percentage.
http://en.wikipedia.org/wiki/Sex