This build is all about hitting things. Fast. By games end, your AS will be north of twice a second, approaching 2.5 when standing in Contaminate.

The Runes are chosen to give additional attack speed and armor penetration. Simple.
The Masteries are mostly defensive to take advantage of additional attack speed, and because the build doesn't really contain any defensive items (excluding Madred's Bloodrazor).

I take Smite and Exhaust for my Summoner spells. Smite for Jungling, Exhaust for some early-ish ganking.

The skills take advantage of Trundle's CC, maxing out Rabid Bite and almost maxing out Contaminate by mid-game. You should only need a level or two of Pillar of Filth early on. It's not a build designed to hunt enemies. If I cannot kill any enemy with a quick Contaminate + Pillar of Filth, using other abilities as necessary, I will give up the chase.

Most, if not all other builds, promote Trundle as a jungler. This is nothing but truth.

Assume you are starting at the bottom base. Head straight for the two trolls just off the bottom lane. If you have any additional XP items (runes, masteries or Zilean), then these two trolls will give you a level, straight out. Make sure NO-ONE HELPS YOU. Then head to the wolves, back to the wraiths, and then back to the trolls. Grab the red or blue aura around level 4, depending on HP and health potions in store.

Use smite whenever you can. Its cooldown is 75 seconds, and gives an extra 5 gold. Over the course of a game, this will add up to alot. It is of particular use when you are down on health, given Trundle's passive ability.

In terms of Items, start off with 5 potions and cloth armor. Your first trip back to the store should net you Madred's Razors. Depending on any success with ganking, also some Boots. Return to jungling and finish off Boots of Swiftness. Then head for Madreds' Bloodrazor and Phantom Dancer. You should be attacking at least 1.5 times a second now. Stark's Fervor is very good because it's lifesteal, and attack speed.

I have only recommended 5 items here because the last is very much situational on the game. If you are taking lots of magic dmg, grad Force of Nature. If it is all physical, invest in a Thornmail. If the other team is all tanks, purchase a Last Whisper for extra armor penetration. If the enemy is really doing much of anything, I'll grab another Phantom Dancer or a Sword of the Divine.