Hypergamy refers to the act of marrying one who is of a higher social class. Women are most commonly noted as doing this, but men can do it as well. There are various reasons for this, and in many cultures it is encouraged by a bride's parents in order to raise the rank of the family. Until recently, hypergamy was considered standard for many women.
The reasons for hypergamy are varied. One of the most common reasons one may choose to marry someone of a higher social class is due to economic hardship. A woman may choose a man who is "well off" in order to remove herself from poverty. In this case, the male suitor would not even have to be exceptionally wealthy. For one who is impoverished, any educated man with a decent paying job would be considered a prime candidate for marriage.
Other times, a woman may engage in hypergamy in order to raise her social status to far above average. This is what is commonly referred to as a "gold digger." The woman or suitor in this case may not be impoverished, but is someone who wants to be wealthy and to have a high level of social affluence. Marrying a man who is much older is common in this situation, and the woman is stereotypically expected to be young and beautiful.
Reasons why women are most noted for hypergamy include the fact that women usually crave security in a marriage, while men care most about physical attraction. While both partners may want someone who is smart, caring, and fun to be around, women are most likely to be concerned with social status and wealth than men. Likewise, a man may be more likely to marry a woman with no social status but an attractive appearance. This will vary with the individual.
In many cultures, hypergamy is encouraged by a bride's parents. This was true of Western nations until recently, and in may societies this is still the general rule. When a woman marries into a wealthy family, the husband generally ensures that the parents are well cared for. This, in a sense, move the entire family into a higher social class. The father of the bride may even have more career opportunities through his newfound connections. Additionally, in some nations the men traditionally move in with the bride's family, thus giving the household more wealth as a whole.