I really like this website for understanding orbs.
http://www.skyscript.co.uk/aspectorbs.html
A lot of good stuff has been writing here... so my two cents is: orbs also are in nut-shell: how much light is shared between two planets, and the kind of light, based on their angles to each other in the sky, also then indicates how strong or bright that light shines, based on the amount of the orb. Also known as the "orb of influence."
It's traditional from what I gather, but to me the logic prevails to this day. Orbs also, I guess, as I have noticed with myself, that you get more intuitive about when reading charts. Its where the mathematics become really important but you ask yourself then the feasibility or quality, character, nature, of what the math is showing you. As was already written, you know it does depend on planets, aspects, so much really.