The document defines a set as a well-defined collection of objects. It provides examples of sets, such as a bag containing pencils and erasers, and explains that a group cannot be considered a set if it is not well-defined, such as a group of beautiful girls where beauty is not well-defined. It also clarifies that a group can be a set, like a group of boys, if the number of members is well-defined. The document notes that individual elements in a set are called members and sets are denoted with capital letters in curly braces.