Investigates topological invariants preserved under homeomorphisms (continuous bijective maps with continuous inverses). Euler Characteristic \(\chi = V - E + F = 2 - 2g\) classifies closed orientable surfaces by genus \(g\) (Sphere \(g=0, \chi=2\); Torus \(g=1, \chi=0\); Double Torus \(g=2, \chi=-2\)). Non-orientable surfaces: Möbius Strip (one-sided, single continuous boundary curve, \(\chi=0\)) and Klein Bottle (closed non-orientable surface with no boundary, \(\chi=0\)).