Yorick is basically a really odd AD Caster. He starts out using his 2 ranged abilities just about exclusively, relying on his ghouls to damage the enemy and wear them down. His 1 level in Q is just in case they try to gapclose with him, which ensures he can autoattack/Q, then have a 3rd ghoul to beat on them and get the extra burst and damage reduction.
Yorick can't really KILL anyone...he just wears them down and forces them back eventually, which is a lot different and more passive than many strong top laners who can get a kill or two and start steamrollering. He gets kills by waiting for stupid people to try to duke it out with him in melee, whereby he tripleghouls/Clones and deals tons of damage with 20% damage reduction.
AD - tons of it. Ghouls have no crit and fixed AS, so AD is the only way to boost their offense. This is why he sucks endgame...other ADs at least have crit to multiply their damage.
Tankiness - his burst damage sucks and scales pitifully, and it takes ghouls a while to wear someone down. So, he needs to live long enough for this to happen. Hence, most of his item budget is in defense.
CDR - After his first 3 ghouls, hes useless due to long CDs. 40% CDR means 67% more ghoul spammage, which keeps his damage going AND his damage reduction up. Helps him chase with Q and slow with W too, and more lifesteal.
Mana - hes a mana hog, thus the use of Tear. Chalice is strong but his mana pool is soo crappy that he still runs OOM in a fight. Philo is strong but not enough regen by far. So tear is perfect, since it allows for manamune to gain tons of AD over time while solving the mana problem too.
That's pretty much it. He avoids Attack Speed, Crit for reasons already listed. He can kind of use both, but not nearly as well as most. So AS/crit he gets are incidental (like from an atma or triforce)
The items that he likes are:
Frozen Heart - mana (for manamune), CDR, armor.
Ionic Boots OR Spirit Visage - for CDR. SV adds more MR/health of course.
21 defense adds 8.1% CDR which will max with a Heart and near-max Boots/SV.
21 offense if getting the magic penetration adds 4% CDR which near-maxes with Boots/Heart.
For magic resistance, SV is good as is Negatron to eventual Banshees Veil (which adds much needed health, and mana for AD).
Triforce is very strong as a late offensive item since he can trigger the damage proc very often and the slow proc helps chase. Movespeed boost is a big boon too. BUT, tankiness is just higher priority.
Warmog is fantastic with his high armor from FH and MR from merc treads/Negatron/SV.
Runes will usually be flat AD red/quint, armor yellow, MR/lvl blue.
19/21 offense allows for the +3 AD, and AD per level, plus both penetrations and some CDR.
9 defense adds some armor/MR/health. 11 allows the anti-minion damage for freezing a lane (though ghouls are good at this too).
You usually want to ult your ADC since cloning him does the most damage. ESPECIALLY if its a true rightclick champ like Ashe, Trist. Cloning an urgot or Ez isn't nearly as useful. Not only is it Life Insurance, but the clone makes it a 6v5.
Sometimes cloning your tanky DPS makes sense if they get bursted down really hard. They will rez and be able to continue providing pressure on the enemy for the entire fight and they are tough enough that they are unlikely to get re-killed.