best explanation is that you have three offensive trees, a utility, and a defensive tree. Don't think of classes (adc, ap, etc) for each tree. Think of individual champion's traits to fit trees. This is why riot allowed is to edit trees in champ select.
#1:You can have a primary tree with all the benefits just for choosing it as the primary tree, this is basically the substitution for main runes like attack speed, dmg, ap... and a secondary tree with two runes selectable out of 9. These are just small added bonuses like masteries.
Primary trees yield passive benefits just for picking the tree. Yellow is attack speed, red is damage, etc.
If you use autos, you run the yellow tree primary.
if you are someone who patrols around and has burst, red primary.
if you are more ability based with higher cooldowns, blue.
light blue aka utility tree is pure situational. It is NOT standard for any champs.
green tree primarily for anyone initiating.
#2:Obviously you'll find that even though you shouldn't classify trees into roles, you will usually end up still pairing yellow with adcs, green with tanks, red with assassins, etc. But I stress that you shouldn't think trees based on classes. Some adcs run red and yellow, some run yellow and green, some yellow and light blue..
#3: The three offensive trees stand out the most, and it's important to quickly learn which of the three work with your champ, since all champs will pretty much take one of these trees, even if you are a support. Light blue and green are usually secondary for most champs because they aren't stat-rich or damage-rich trees.
This system is super complicated but it's just a matter of knowing each individual champs strength with the primary trees. Again, don't think of it as adcs run yellow, assassins red, mages blue, tanks green... Because there are many exceptions. Like I think draven runs red primary well, ezreal runs blue well, etc.
This post was edited by EnigmaSeven on Nov 24 2017 07:40pm