A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs).
These antigens may be proteins, carbohydrates, glycoproteins,
or glycolipids, depending on the blood group system.
Some of these antigens are also present on the surface of other types of cells of various tissues.
Several of these red blood cell surface antigens can stem from
one allele (or very closely linked genes) and collectively form a blood group system.
Blood types are inherited and represent contributions from both parents
The two most important ones are ABO and the RhD antigen;
they determine someone's blood type (A, B, AB and O with + and - denoting RhD status).
Many pregnant women carry a fetus with a blood type different from their own,
and the mother can form antibodies against fetal RBCs.
Source: http://en.wikipedia.org/wiki/Blood_type
Blood types in Korean and Japanese culture
There exists a common, popular belief in East Asian countries
that a person's ABO blood type is predictive of his or her personality, temperament,
and compatibility with others, similar to how astrological signs are used in other countries
throughout the world, although blood type plays a much more prominent role
in East Asian countries than astrology does in other countries' societies.
Following are the personality traits ascribed to the basic blood types:
Blood Type A:
Positive Traits: Conservative, introverted, reserved, patient, punctual and inclined to be perfectionists.
Worst Traits: Obsessive, stubborn, self conscious and uptight.
Referred as ‘farmers’ in some descriptions, Type A’s are said to be considerate of others and loyal to a fault. They can also be secretive and reluctant to share their feelings. Apparently they don’t hold their liquor well, either.
Blood Type B:
Best Traits: Animal-loving, creative, flexible, individualistic, optimistic and passionate.
Worst Traits: Forgetful, irresponsible and self-centered.
Referred to as ‘hunters’, Type B’s have very independent natures and tend not to be concerned about what other people think of them. Although often described as shallow and lazy, they can be quite passionate about the things they hold dear. Patience is not their strong suit either.
Type B men have acquired a very negative reputation in Korea and are not considered by many to be good husband material. Often described as ‘players’, they are perceived as being selfish and mercurial, quick to anger and not terribly reliable. That said, their bad boy image makes them very attractive to women, but not for the long term. (Type B women do not share in this bad rep, for some unexplained reason).
Blood Type AB:
Best Traits: Cool, controlled, empathic, introverted and rational.
Worst Traits: Aloof, critical, indecisive and unforgiving.
Referred to as ‘humanists’, Type AB’s are said to be controlled more by their heads, than by their hearts. They are rational, good with money, but unpredictable. Although inclined to be distant, they prefer harmony and as such, work well with mediators. Some consider them two-faced, and therefore untrustworthy.
Blood Type O:
Best Traits: Ambitious, athletic, robust and self-confident.
Worst Traits: Arrogant, insensitive, ruthless and vain.
Referred to as ‘warriors’, Type O’s are viewed as natural leaders and are often, also, natural athletes. They tend to be outgoing, expressive and passionate, but can also bore others to death with their obsessive drive for success coupled with their absolute convictions that they are winners. This certainty that they will always win explains why they aren’t afraid to take risks or gamble. They have a strong physical presence and are unlikely to ever be overlooked.
Source: http://en.wikipedia.org/wiki/Blood_types_in_Japanese_culture
http://www.korea4expats.com/article-blood-types.html