These are first-person plural personal pronouns. We is in the nominative (or subjective) case; us is in the objective (accusative) case. Choice between them depends upon the function each fulfills in a statement: "We taxpayers are entitled to a referendum." "For us taxpayers the outlook is dubious." After as or than in a comparison in which the first term is in the objective case, use us: "The waiter gave them more food than us." Conversely, write "Those men are taller than we" because we is understood as the subject of the omitted verb are, exactly as us in the immediately preceding sentence is the object of the omitted verb gave. In addition to us and we, there are five other regularly used personal pronouns with nominative and objective cases: I and me, he and him, she and her, they and them, who and whom. These pronouns should always be used in their nominative (subjective) forms except when they are (1) the object of a verb, (2) the object of a preposition, or (3) an indirect object: "I love her." "She loves me." "He hates him." "They have come." "I saw them." "Who is that?" "Whom did you see?" "I talked to her." "I gave him a hug."
Dictionary of problem words and expressions. Harry Shaw. 1975.