Slurpees are 7-Eleven's version of microspheres of ice mixed with some soft drink syrup to give a tastey treat. In Canada, Mac's Milk also has this concept but they call it a Froster. These are all analogous to the original slurpie, with that weird white dog sucking down odd fruity flavoured ones, kinda like the snow cones you get at carnivals. I believe they are called Slush Puppies hence slushie. I personally like Coke Slurpees best, but the consistancy has to be correct. The key to having a good slurpee is to make sure that the ice is well enough frozen. If the microspheres are not in great quantity, you will mostly have flat, throat burning syrup/water. It is yucky. One can't really refreeze them either as the water has a higher freezing point than the syrup hence at 0 degrees C, they separate.
If you live out in the boonies and can't get any of these brand named versions, make your own! I do often. The best way to drink Coke, for study purposes, is to freeze it just enough that the top half inch of the can has ice crystals/chunks. This will increase your metabolism as the body will have to strive harder to get your overall temperature up after drinking it. If you want to go further and make a full slushie, then take 8-12 standard icecubes, crush them up and mix in with your favourite soft drink. Make sure that the liquid is nearly the same temperature as the ice, so that the ice does not melt as quickly. Although this is not as great as the store-bought ones, it is close and worth a try.