Seeker Chariots of Slaanesh
When the Daemon armies of Slaanesh need something with a little more oomph to their cavalry forces than Seekers of Slaanesh, they hitch two Steeds of Slaanesh to a chariot. When ridden by a Herald, it is called an Exalted Seeker Chariot of Slaanesh.
Also called "Shrieking Shandredhans", "Glorious Doomriders", and "Pallid Vanguards".
Seeker Chariot of Slaanesh
The chariot is designed to be "appreciated" in motion as an abomination of shifting shapes due to the bizarre structure of the wheels and the polished metal of the vehicle itself casting reflections and shadows that strain the sanity of the viewer. The axles cause a screeching noise that is appreciated by Daemonettes, but is a nightmarish cacophony to everything else (so punk rock wheels). The Daemonettes aboard are comfortable with the movement and literally dance back and forth as the cart jostles across terrain, slashing at foes and enjoying the ride every inch of the way. The spiked bottom of the cart and the blades on the wheels turn the vehicle into a thresher that runs over foes.
On The Tabletop
Seeker Chariots are an upgrade from Seekers of Slaanesh, and aren't special enough to really merit much discussion otherwise. More expensive in points, slightly more killy, fulfills the same role. Take it if it suits you, or skip it in favor of Fiends of Slaanesh the same as you would Seekers.
Seeker Chariots are very cheap suicide units. They are fragile to both ranged and melee, but they shine when making Hammer of Wrath attacks. If you manage to pull off a charge, they can do some serious damage. They are also one of the cheapest way to complete the Grand Cavalcade formation, which buff their HoW attacks even further. Just don't expect to be able to pull off a charge all that often, they are more like a mini version of the Distraction Carnifex.
A cheap and convenient way to deal Mortal Wounds, but only against infantry (and cavalry with smaller bases). Since you don't generally need Mortal Wounds against mass infantry, Chariot really shine against elite mono-wound infantry. Don't be afraid to retreat them from combat to use their impact ability again, they really don't deal that much damage in continuous combat. If you need a fast Hero to complete your army, a Herald on Seeker Chariot is quite a good solution. They are fast, cheap, versatile and their big base will give you quite a good Locii triggering range.
Same as Fantasy, take them as an alternative to Steeds. They got even more destructive with a buff however, but as they only exist in a Unit of one they actually become the superior Hammer than a Unit of Seekers. If the Fast Cavalry rule is what you need, or if you intend to use a larger Unit or a Herald in a bunker, go with Steeds. If you need something that hits hard with some staying power, use Fiends. But if you need something fast that can bring the pain, Chariots are the way to go.
Exalted Seeker Chariot of Slaanesh
Exalted Chariots are Seeker Chariots that carry a Herald of Slaanesh. They are much larger (literally being made of two Seeker Chariot model kits), with four Steeds and three Daemonettes as crew while the Herald brandishes two whips from the elevated platform on the back. The thresher is expanded, and now looks like it could literally just run over a unit of almost anything.
Heralds will pluck victims from the masses running from the blades to...enjoy...in vile ways while the battle rages if they get bored going at several hundred miles an hour in an open vehicle.
On The Tabletop
While a bit of an investment, and certainly cannon-bait like everything fun you can field in Fantasy, an Exalted Chariot is a fucking BEAST. It literally will eat an entire Unit. Unfortunately, they tend to become a centerpiece model as they REALLY eat the Rare points. Still always worth considering.
Now called a "Shrine Of Temptation", it has gotten a buff to offensive capabilities and a nerf to defensive stats. It also has a nifty new ability that grants ASF to your Slaaneshi forces in the same Combat, making this model even more of a glass cannon of a hammer than it was before. Still worth it, but remember that its going to take some serious flak.