Two kinds of power prevail in Steinbeck's book - material/financial power and physical power. These forms have their counterparts in the powerlessness of poverty and physical disability.
Many of the characters in the story are directly aligned with one of these categorical descriptions. Slim presents an example of how physical prowess translates into esteem within the community of the ranch. He is very capable as a skinner and this physical ability makes him the most respected member of the community.
Emphasis is placed on Slim's skill and craftsmanship; he does his job exceedingly well.
Contrastingly, Candy and Crooks have physical disabilities that can be read symbolically. A missing hand and a crooked back are representative (and perhaps the cause of) the powerlessness that characterizes them. With a diminished ability to do work, Crooks and Candy are relegated to the low end of the social ladder.
In these ways, power is presented in a rather literal way in the book. Social power grows from physical power.
Financial power is another mode of power in the book. Curley and his father occupy a position of power, with the ability to hire and fire men. This type of power is not enough to satisfy Curley, as we can see from his behavior toward Lennie. In picking a fight with Lennie, Curley displays the nature of his power as well as his insecurity.
His position in society has encouraged this behavior; his real strength lies not in his fighting ability but in his power to fire any worker.