Why would you want to play Yorick?
Yorick is a great Split Pusher with a strong Ability Set.
Yorick is easy to learn.
Yorick is good against immobile Champions.

Pros:
-Great Split Pusher
-Easy to play
-Can easily 1v1 almost any Champ
-You can be useful without depending on your team
-You can Push multiple lanes at the same time (with Maiden)

Cons:
-weak early on
-not much mobility
-W is not that easy to hit most of the time

Passive: Shepherd of Souls
Yorick raises a grave for every enemy champion, large monster and 12th / 6th / 2nd non-champion enemies that dies near him, or for any enemy he kills with Last Rites.

These Graves are diplayed on the ground. The mist walkers can be raised drom their graves when at least 3 Graves are present near Yorick. Yorick can only have 4 Mist Walkers up at a time.

Q: Last Rites
When using Last Rites your shovel empowers your next AA within 6 seconds to deal bonus physical damage and also heal Yorick. If Last Rites kills an enemy a Grave will be spawned.

When there are enough Graves nearby (min 3, max 4) you will be able to raise the nearby Mist Walkers with Awakening, this will consume the Graves.
Last Rites will also reset the AA timer.

W: Dark Procession
Yorick summons a cirlce around the target area after a small delay which stays for 4 seconds, depending on the Level it has 2-4 HP and it can be attacked my enemy Champions, it behaves similar to a ward, where 1 hit takes away 1 HP.

This will be your trapping Ability, it can help you catch up to your enemy or prevent them from running away, it can also help you to hit your E: Mourning Mist.

E: Mourning Mist
Yorick hurls a glubule of black mist at the targeted location, dealing 15% of target's current health magic damage to all enemies within that area, slowing them by 30% for 2 seconds and marking them for 4 seconds, this also reveals champions.

Marked enemies take 40% more damage from the next 8 Mist Walker attacks, Mist Walkers will also jump to your target and hit them.
Mist Walker and the Maiden gain 20% movement speed toward marked enemies.

Marked Champions raise nearby Mist Walkers from Graves, which will attack them.

R: Eulogy of the Isles
Yorick summons the Maiden of the Mist, along with 2 / 3 / 4 Mist Walkers.
The Maiden can move and attack on her own, she deals magic damage and raises 2 / 3 / 4 Mist Walkers from nearby enemy deaths.

This ability can be recast allowing you to send The Maiden down the nearest lane making it possible to push 2 lanes at the same time as Yorick.
The Maiden will stay up until she dies (staring the ability cooldown), but she can regenerate health in the fountain just like a champion.

This is the ability that raises your Split Push potential even more.
Important Note: The Maiden will follow you, but you have to be careful beacause once she attacks something you will have to get quite a bit away from her to start following you again.