A set is a well-defined collection of distinct objects. Notation: roster form A={1,2,3}A = \{1, 2, 3\}A={1,2,3} and set-builder form A={x:x∈N, x<4}A = \{x : x \in \mathbb{N},\, x < 4\}A={x:x∈N,x<4}.