It mainly relates to shape, and therefore the flying characteristics that shape imparts on the kite.

These days, except for pure high end "C" kites, references to SLE, C, Bow etc are all really more references to the flying characteristics a particular kite has. Many now are hybrids with elements/flying characteristics of a number of classic shapes.

To give it perspective you have to understand the sequence kite development followed. I may be slightly wrong but generally:

C kites were the originals, shaped pretty much like a beach towel held at either end, and forming a C.

Then Bow kites were patented which had a radically different shape, being flat and swept back. That gave rise to discussions on C v Bow both of which had very different flying characteristics (and quirks)

Then came SLE - Supported Leading Edge - kites. These sat in the middle a bit, although still more bowish. They removed a lot of the bad elements of Bows but kept many of the good aspects.

Then came the hybrids and Deltas - both further mixing and blending the best characteristics that had C's at one end and Bows at the other.

Ultimately C kites these days are specialty kites generally used by advanced kiters looking for the attributes they provide. Generally consistant smooth power, rapid response, insane kite loops, ability to depower through edging (not bar throw). they are not the best to learn on.

SLE is rarely used these days as far as I can tell as Delta and Hybrid's generally cover the mainstream kites. Technically, any kite with a bridle that has multiple attatchment points on the leading edge is an SLE though.

Deltas are known by the shape - mainly pointy tips, giving the kite a delta shape in plan. They have mainly taken over the SLE field and have static power, bar throw (safety) good relaunch, but can vary a lot really.

Hybrids try to move the performance close to a C, but not lose some of the other nicieties SLE's had. Generally no bidles, and a very C shape to the canopy, but still curved LE rather than staight and a LE front line attachment rather than the square ends C's have.

Ultimately there is a lot of blurring nowdays (apart from true C's) and no discriptor can really be applied accurately to a group of kites because of it's shape. The references are really only good to try to discribe the characteristics of a particular kite in the absence of anything else.

Just to muddy the waters a little.

Bow Kites - patented by Bruno Leganioux and have 4 distinct features

SLE Kites - Includes BOW kites, but also plenty that don't fit the patent, some part of the design has been changed. (Originally the trailing edge was not concave)

The terms SLE & Flat kite were marketing terms used to describe kites that couldn't be called BOW due to the patent.

Kiters owe a lot to the Leganioux brothers. (And Peter Lynn and a few others) They've got a website here if you want to read more:
http://inflatablekite.com/en