Raytracing
Raytracing is a rendering technique in computer graphics (CG).
The basic idea of raytracing is to reverse the process of real-world light propagation into our eyes and instead trace back the path that the light might take from our eyes back to the light source.
The much more common rendering technique in CG (especially in real-time rendering) is rasterization.
How it works
In the real world, we visually perceive the world by sensing photons that enter our eyes. The bulk of these photons come from light sources such as the sun, although they could have bounced off of several materials multiple times before reaching our eyes. The basic idea of raytracing is to reverse this process and follow all the paths (including all indirections) that the camera could take to all photon emitters in the scene, such as a lamp or the sun. If there is e.g. a path from the camera to a blue vase to a point light, we could color the pixel on the screen that corresponds to that path blue.
Pathtracing vs. Raytracing
Path tracing is a more advanced form of ray tracing, where instead of tracing a path from a surface directly to every light source, the ray gets bounced off in a random direction until it hits a light source or exhausts the bounce limit.