A Splined hub is a hub that has āsplinesā on the axleā¦these are like a number of little grooves that make the hub a lot stronger by increasing the surface area of the axle, allowing maximum surface area therefore maximum grip with the cranks.
The splines are like little rises and runsā¦you can get cranks from about 8 splines (8 rises/runs) to about 36 splines (36 ārises/runsā but with so many look like little spikes kind of)
its kinda hard to explain but I hope this helps. On www.krisholm.com on the products page have a look at his hub/cranks and you can see what a splined hub is, and see what i mean about the little groove.
Oh and basically splined is an alternate method to square cotterless cranks. which are square so only have four sidesā¦not much surface area - not very strong.
ok i get it now 
Coolā¦'cos reading back I hardly understood what I wrote:D
EDIT: Im sure someone else will be able to add something a little more technical and easy to understand then what I wrote.
In addition to more surface area in the hub/crank interface, splines also also allow for much larger diameter hubs. This greatly helps with strength.
Splined cranks come with up to 96 splines (profile SS, or is it Dirtjumpers?). Unicycle hubs come with anywhere form 8 splines to 48 splines. KH old: 8 spline. Qu-Ax: 10 spline. KHnew/Onza: 36 spline. Old Onza: 40 splines. Profile: 48 splines. KH old versions have huge splines, about 1/8" deep or so. Profile splines are about 1.16-1/32" deep, but far more numerous.
Yo Homie itās like this. There be 3 major types of crank to axle interfaces: Cottered, Square Taper, and splizined.
Cottered:
Iāll let my man Sheldon Brown Handle this one:
The axles and crank holes are round. The axles have a groove in them. A wedge, called a cotter, slips in there to keep the crank still.
The cotters are thin little pieces of metal that taper. They wear out and have to be replaced. Sometimes they shear in half and bad times. Donāt get those.
Square Taper:
Axle:
Cranks:
This was the standard in the bike industry for many years and is still the standard in the unicycle industry. Many bikes still use this as well. The axle is a square peice of metal that tapers (thinner on the outside). This allows you to press a crank (which has the same taper on the inside) on to the axle by using a bolt. This system is reasonably strong. Problems exist in the maintenance. The cranks cannot be removed very often, since each installation removes material from the axle. This will eventually wear down your axle. On bikes it screws up your chainline. Also, if someone rides on a loose crank, the axle and crank turn into round useless things never to be useful again. If installed properly the first time, this is a good system.
Splizined!!!
Axle
Crank
A more recent development in the bike industry and even moreso in unicycles, splined axles are totally the way of the future dude. Theyāre like mega awesome. The axle has grooves on it. The inside of the crank has a matching pattern (although different companies make different patterns, because unicyclists are dumb and donāt use ISIS yet) and they slide together all nice like. Itās great, thats all you need to know. Buy some. And then bitch that they arenāt ISIS.
Donāt forget about the ever popular single piece. But, donāt worry about that. Itās not used much in unicycles, and the ones that do use the are crap. They are mainly used on kidās and BMX bikes.
Technically I did not forget about it, since it is not a crank to axle interface. I suppose you could argue that I forgot about 2 peice cranksets, but those are gay and useless also.