If love is the central feature of Western relationships, 정 (jeong;情) is its equivalent in Korean culture. It's a difficult concept to define or explain because nothing like it exists in the Western world. English words that come close to it include relationship, ties, bonds, familiarity, rapport. Now, imagine that those ideas are the most important overriding concept in your culture, stronger than love, and you're getting the idea.
[edit] 정 and Love
정 is often compared with Western love, but it is not the same as love. While in the West, we believe that love is the glue that holds marriages together, in Korea, it's 정. One might say to a spouse, "I may not always love you, but I can never leave you because of our 정." There is no equivalent concept of having instant 정 with somebody in the way that we claim to fall instantly in love. Accumulating 정, by definition requires a lot of time. There is no idiomatic equivalent to falling in love that is used to express a critical mass of 정. A major difference between 정 and love is that hatred and 정 are not mutually exclusive. In fact, hatred entails 정: if you spend a lot of time with somebody, and you hate them because they do things to hurt you, the very reason it hurts is because of the 정. If you didn't know that person, and had no prior relationship with them, it wouldn't hurt. It would piss you off, but it wouldn't hurt. That's 정.
[edit] 정 and Social Functions
정 has many social functions outside of significant relationships. In Korean markets, some haggling is allowed, not so much to give the customers a better deal, but to cause interaction, and thus create 정. In friendly little market streets, the ajummas will often try to give you little free things. If they give you something, you will like it and it will make them feel good. It's been described as the 정 economy -- you make somebody feel good, and it will make you feel good too.
정 also explains why pushy ajummas on subways can get away with it so easily -- the young people they're pushing around know that under different circumstances, that same "rude" woman would be just as likely to give them a candy.
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