Korean people are quite strong believer in the theory that person's personality is affected by blood types. In western culture, it is not so common to know what their blood type is, but in Korean, I am sure that "EVERYONE" knows what their blood type is. As far as I know, the theory is NOT on the scientific basis, but how Korean people feel is that it's correct in a way, and it's not always correct in other way. How strong beliefs each person has is really each to their own.
Here are the general summary of characteristics according to each blood type:
1. Blood type A- Known for being timid, sensitive, passive, reserved, easily mad or sulky from tiny things, etc.
2. Blood type B- Known for losing their temper easily, being outgoing, passionate, active, cool, witty, etc.
3. Blood type O- Known for being easy to get along with, easy going, passionate, social, etc.
4. Blood type AB- Know for being peculiar, extraordinary, etc.
If you are in the stage that you started to know and see someone, the blood type is the one you might definitely wanna know and check in Korea, even for fun. And sometimes people tend to talk about their personality using their blood type. For instance, you disappointed somebody by mistake, then the person might say "나 A 형이야..." which means "I easily get hurts and I don't normally reveal what I really feel to anybody"