Sets are collections of well-defined objects that can be represented symbolically. A set contains elements and its size is represented by a cardinal number in curly brackets. There are different ways to represent sets, including roster form which lists elements and set-builder form which uses properties to define elements. Some common types of sets are empty/null sets containing no elements, finite sets with a limited number of elements, and infinite sets with an unlimited number. Examples of standard sets used in mathematics include the sets of natural numbers, integers, primes, and rational numbers.