Originally Posted by PogoPogoPogoPogo
While the previous post is a pretty good explanation on the differences, the tips offered at the end are a little off in my opinion.
Thanks for cutting me some slack, it was like 3 in morning I decided to finish that. But yes, Pogo's right here, taking Flat AD in terms of a ADC is very good for last hitting. However if you want to play other champions who...
A, have a strong lasthitting ability
B, have very high base damage
C, are in the jungle early game
You want to look at their kits and make sure that the runes you take synergize with their other abilities to make the most of your runes early game, or lategame, if you're into that sort of thing.
Remember also, to decide whether or not your champion shines late game or early game, lets look at AD Kog'maw and Urgot for a sec.
Both have extremely long range abilities, high physical damage, an armor reduction ability, and a kite-friendly slow.
Urgot will obilerate enemies in the early game, his laning used to be (And still is) one of the most unshakable in the game simply because he could hit enemies from across the map with Acid Hunters. His range has been nerfed, but the point is, Urgot is a champion who needs help in the Lategame, because his early game is devastating by itself without runes.
Kog'maw on the other hand is a carry, and tends to have low(ish) early game damage and depends on his team for protection, however, once he gets fed he becomes a devastating killing machine, being able to raze even the toughest champions in the game very quickly. So you would want to make sure his early game is set up to get him into his lategame so he can really shine. Some defensive Runes and items early can help him out with this.
So bear in mind how you play your champion, and what their strengths and weaknesses are, and choose your own runes accordingly.
On a side note, you can also just pull up a guide on a champion you like and figure out which runes people seem to like, you don't even have to read the entire thing!