This document discusses graphs and Euler paths/circuits. It defines what a graph is - a collection of vertices connected by edges. An Euler path uses every edge once, starting and ending at different vertices. An Euler circuit is a closed path that uses every edge once, starting and ending at the same vertex. The Euler theorem states that a connected graph has an Euler circuit if every vertex has an even degree. The document provides examples of Euler paths and circuits and applications to problems like planning a bike route that uses every trail once.