An undirected graph is a collection of points, called vertices, connected by lines, known as edges. In this type of graph, the edges do not have a direction, meaning that the connection between any two vertices is bidirectional. This allows for a simple representation of relationships where the order does not matter, such as friendships in a social network. In an undirected graph, if there is an edge between vertex A and vertex B, it implies a connection that can be traversed in both directions. These graphs are commonly used in various fields, including computer science, biology, and transportation, to model networks and relationships.