What exactly are "direct lighting" and "visualize particles from light"?
@ak-47: Direct lighting means you shoot a single ray out from the eye, find where it hits the scene, and then immediately connect that point to the light (assuming you know where it is!), with no further bounces. "Visualize particles from the light" means you shoot a single ray out from the light, find where it hits the scene, and then immediately connect that point to the eye (assuming you know where it is!), with no further bounces. In the case where the light is not a point light or the eye is not a pinhole camera, you still have to randomly sample which point on the light or eye, but the path length is still just two in either case.
How is direct lighting different from path tracing then? Is it just that we've eliminated chance of doing more than one bounce?
Path tracing is an algorithm; direct lighting is a phenomenon. For instance, one could compute direct lighting by applying (bidirectional) path tracing, connecting each eye path of length one directly to the light. Alternatively, one could use rasterization to evaluate direct lighting (for sufficiently simple light sources, and ignoring occlusion).