A good text will provide you with extensive technical descriptions of similarities and differences which are way beyond the scope of forum help.
Your example: "Does anyone know what it was about; who did it, and why?" The semi-colon shows a big shift or contrast, from "what" to "who." The comma shows a lesser shift, moving the focus from "who" to "why" (that person's motives.) There may be "rules" but a sense of rhythm and contrast are what's important.