This build uses a combination of mana-based damage, procs, and blend of health and resistances to make Yorick greatly excel in matches.

Mark of Desolation: Best AD runes around. Yorick's a tanky AD that can burst down targets pretty well, and Desol. runes will help him greatly!

Resilience/Warding: You're a melee fighter and will take lots of heat by ranged champions. The reason for not investing 21 in Defense is that Yorick is extremely mana-hungry, and the Utility line and in-game mana items are more than enough to satisfy this issue. Yorick also doesn't need the entire 21 Defense because Yorick's not a full-blown tank. He's a beat-em-up Bruiser type with some amazing burst damage. He only needs enough to keep him alive for them kills and/or assists. He'll have a great early game with this configuration [if played right!], no doubt. The 9 points in Defense above will give Yorick the regen he needs combined with healing potions to stay in lane for a fair amount of time, provided you don't throw yourself into trouble without your laning partner.

Fortitude: You're a melee-ranged fighter, and needs that extra health to last long enough to get that first blood. Your Strength of Spirit investment combined with resistances and these flat health runes will guarantee you some staying power, but don't go wild and 1v2 [unless they're directly under your turret hitting you... ofc].

21 Utility for mana, mana regeneration, skill cool down reduction, summoner spell cool down reduction for Flash and Ghost [escape more, feed less], and longer-lasting golem buffs! <3

9 in Defense for some nice resistances and regen [especially for Yorick's early game!].

Mercury Treads: You don't need the extra move speed as your Q already gives you that [provided that your Ghoul doesn't get killed]. You also don't need Berserker Greaves as Yorick's not an auto-attack carry. Ninja Tabi's block chance is negligible, and mages are of higher concern than auto-attack carries because they can wear you down at range [unless dealing with ranged carries like Ashe... in which case the tenacity from Merc. Treads will actually help you out more than Tabi anyway].

Manamune: Combined with the investment in Utility [see above], Yorick won't run out of mana as fast as he would without. This item lets Yorick build around items like Frozen Heart, Trinity Force, Banshee's Veil, to make him strong. Yorick only needs a decent amount of damage from Manamune and those mana items above to make him a bursty power-house!

Trinity Force: This item [while expensive] is a worthy investment. Amplifies damage, snares when you land proc's, and speeds you up even more. The best off-tank item ever!

Frozen Heart: More mana and cool-down reduction, synergizes well with Manamune for more damage, and most importantly, it slows auto-attacker's attacks down! Slowing down auto-attacks of carries is a form of damage mitigation [especially in team fights!].

Banshee's Veil: More mana for your Manamune to boost your DPS, more health, magic resist, and most importantly, it blocks one spell per 45 seconds. It's an amazing item, and totally worth the investment. Just ask anybody!

Spirit Visage: More MR and Health, more cool-down reduction, and it makes your E give stronger self-heals. Combined with Frozen Heart, you'll now have 40% CDR so you can do without the blue buff [esp. if your allies *esp. if Anivia* needs it more!].

Q - is your bread and butter. Activate it, auto-attack an enemy champ, and continue auto-ing him while your ghoul beats on him for more damage. You can also last-hit minions [esp. cannon minions because they're worth the most!].
E - Use this skill to discharge a snare. Simply fire slightly ahead of wherever your enemy champ target's about to go, fire away, and slow him down. Combined qith your Q and E, your target's now being beat down by your ghouls, and you're also moving fast while your Q Ghoul's alive.
R - Use this if you're:
a.) about to die,
b.) your ally's about to die,
c.) You're about to finish off an enemy champion,
d.) or want to take down a turret quickly [make sure you have enough time to do this. If turret's low on health, and no enemy champs are around, then use it and speed up the process!].

Flash - Yorick's "Get out of Jail Free" card. Combined with Presence of the Master [Utility], you can flash a lot more often, making you feed less and even tower-dive for a kill.
Ghost - While you do have a speed boost from your Q, if they kill your Ghoul off, you'll lose that speed. Ghost is a more solid and reliable way to catch up with enemies.

Exhaust - It's definitely a great spell to have. Yorick can seal the fate of an enemy champion if he uses it while training an enemy champion with his Q-W-E[and even -R].

Ignite - Helps finish off annoying champions like Warwick, Vlad, Nidalee, etc. No mastery investment required as Yorick's not an AP champ .

Teleport - It's a decent spell. Works best if you can set up a pre-made team and synchronize appearing at enemy turrets for quick and collective take-downs.

Clairvoyance - Can be useful for:
Finishing off an enemy champion that's trying to blue-pill out at low health. You can keep track of Golems, Dragon, and [most importantly] Baron!

Heal - Not the best option, but it does synergize with Spirit Visage. It's better if you let your Soraka/Sona/etc. handle the healing while you concentrate on the killing.

Smite - While it can benefit you in lane, it's definitely not the most optimal. Best left for 100% junglers, tbh.

All other summoner spells not mentioned yet - Revive, Fortify, Rally - aren't worth mentioning, tbh. Most players know by now that they're nowhere near as useful as the above spells.

In summary, this build gives you the potential to score early kills, farm, not run out of mana as much, and all while having some serious burst damage. You'll have so much damage, mana, and cool-down while being tanky that you'll be able to stay out in the battle field a lot longer to take down those turrets, inhibitors, and win the match!

Pros:
1.) Beefy!
2.) Good Burst Damage!
3.) Tons of mana!

Cons:
Not a carry...
Not a tank...
Can run out if you spam skills too much [esp. during your early, and even mid-games]...