What is the Bloch Sphere?

Introduction to Quantum Computing

Geometrically, θ and φ can be mapped to a point on a sphere which is referred to as the "Bloch sphere", using a spherical coordinate system. 

The angle θ is called the "polar angle", and it is measured from the positive Z axis to the Bloch vector representing the state .

The angle φ is called the "azimuthal angle," and it is measured from the positive X axis to the projection of state |ψ>  onto the X-Y  plane.