Euclidean geometry is a branch of mathematics that studies the properties and relationships of points, lines, angles, and shapes in flat, two-dimensional space. It is based on the work of the ancient Greek mathematician Euclid, who wrote a series of books called "The Elements." This geometry is characterized by its use of axioms and postulates, which are basic statements accepted as true without proof. In Euclidean geometry, key concepts include the notions of congruence and similarity, which describe how shapes can be identical or proportionally scaled. The geometry also explores the relationships between angles, such as the sum of angles in a triangle always equaling 180 degrees.