Wave optics is the study of the wave nature of light. Interference and diffraction are two main phenomena giving convincing evidence that light is a wave.
Wavefront: A wavefront is a surface joining the points of the same phase. The speed with which the wavefronts move away from the source is called phase velocity or wave velocity. The wave transports energy along the lines perpendicular to the wavefronts, with wave velocity. These lines are called rays.
For a point source, the wavefronts are spherically extending in three dimension space. If the source is far off, the wavefront becomes almost a plane.