The symbol \sim is used as a shorthand to the indifference relation: x\sim y \iff (x\preceq y \and y\preceq x), which reads "the agent is indifferent between y and x".
The symbol \prec is used as a shorthand to the strong preference relation: x\prec y \iff (x\preceq y \and y\not\preceq x), which reads "the agent strictly prefers y to x".