How does raycast works?

I'm a bit confused in general with raycast works and how the ray shows and such. Wiki kind of confused me a bit. Can you explain how it works in a simple way?

From my understanding of Raycast (and it may not be that much, or an adequate answer at all), it draws an invisible line between point A and point B. That's really it I think, but you can still do things with it like RCL gun scripts or whatever.