Structural functionalism and conflict theory are two of the three main theoretical perspectives in the field of sociology. You can read detailed discussions of both perspectives by following the links below. The two perspectives are different because they have different ideas about how the various institutions and customs of a society come to exist.
Structural functionalism tends to see society as an organism. In a living organism, every part of the body has some role to play. All of the body’s parts help to keep it living. Functionalists say that society is just the same. All of the institutions of our society have, in essence, evolved to keep our society stable. For example, our system of marriage has evolved, you could argue, to provide an outlet for people’s sexual needs while, at the same time, ensuring that children will be provided and cared for. Thus, marriage helps to keep society stable.
The conflict perspective, by contrast, believes that society is created by conflict and struggle. From this point of view, various groups in society are constantly competing for power and resources. Conflict theorists would say that the institution of marriage arises out of conflict between men and women. It creates a system in which men are given the upper hand. Women, for example, tend to do most of the work of keeping the home and raising the children even when women work outside the home. Men are traditionally seen as the head of the household and generally have more sexual freedom than women. In short, marriage has evolved out of gender conflict as a way for men to maintain their power in society.
Thus, structural functionalism and the conflict perspective are very different in terms of how they account for the characteristics of human society.