Exactly what the name implies; a thowing blade that resembles a playing card, i.e. poker card, face cards, etc. The resemblance is only superficial, though, since metal is very hard to shuffle...
In the intrepid days of my youth, I fancied myself as a bit of a metalworker. I really wasn't, but I fancied myself as one. One of the things I fashioned (and the onlything that worked) was the erstwhile Playing-Card shuriken.
I got the idea from an episode of some animated Batman series where The Joker had a metal playing card (the Joker, of course) up his sleeve and threw it at the Dark Knight. It seemed like a cool idea, so I tried it. A made a few of them over the years, each one getting successively better than the last.
The first, many years ago, was made out of a piece of HVAC ductwork. If I had known at the time, I would have used much thicker metal, but this was a first try, so I didn't. I traced the pattern and cut it out with tin snips. Of course, the act of cutting bend the metal, so I had to flatten it again, and afterword a went about sharpening the corners with a bastard file. It was a very time-consuming process.
It worked somewhat well, but the thin-ness of the metal affected it's performance -- the corners dulled quickly and wrinkled if it hit anything harder than wallboard/sheetrock/drywall and it was so light that it couldn't keep the correct attitude for a long distances.
The second try was out of thicker (but still kind of thin) steel pulled from a some construction site. I still used the tin snips, but this was thicker metal so it didn't distort as badly. The key difference here is that this time I used a bench grinder to sharpen all the edges (not just the corners). I also gave this one a very spiffy paint job that make it look quite like the Ace of Spades.
This one bit into things very well and could anchor itself in a tree quite well, although the weight and balance was still a problem. It would travel straight if thrown vertically. If though horizontally, it would catch the air wrong and spin out of control. I made a few of these, testing out various sizes and paint schemes.
The next, and still the best I think, is a metal that really looks like stainless steel. I found it on a roadside already in the shape of a rectangle, though about 50% larger than your average playing card. It was a very heavy gage, but still light enough to be agile. I again sharpened all four sides with the grinder which, due to the metal, gave them mirror-like edges.
This iteration is a beauty. The perfect weight, and just a little large, but not overly so. It keeps and edge very well and has the weight to bite into anything save metal and stone.
I have since been working on another. This one has been cut from a steel sheet with a MIG Welder, though it has been harder to put an edge on to. Not finished yet. Probably never will be.
Things to remember. These are not traditional shuriken; they are an odd shape and as such have 'flat' sides and such has a chance of not biting into the object in question, but instead bouncing off.. though if they happen to hit a person at a speed like that, even and flat side hit will cut flesh. Be careful.