The grind refers to how much metal that has been 'scooped' or ground out of the side of the blade. If you look at the blade face on (in profile) the grind can be studied - it is the hollow area on either side between the top of the razor (the spine) and the cutting edge. There have been a number of systems to define grinds, some quite confusing. Basically, a true wedge razor will have no hollow grinding - it will look triangular in section, the sides being true straight lines. These are often quite early blades, and are very time-consuming to hone because metal has to be removed from the complete face of each side of the razor. Chiefly in an attempt to ease the honing process, the hollow grind was introduced - this meant that only the spine of the razor and the cutting edge rested on the hone, meaning much less metal has to be removed to sharpen the razor, making the whole process quicker and less arduous.
At the other extreme we have the full hollow ground razor, also known as "klang" or "singing" - klang is german for sound, and this type of razor supposedly has such a thin blade that it can be plucked to produce a ringing note. The Henckels grind table shows 16 variants starting with the wedge and ending with full hollow, but the differences in shape, particularly at the full hollow end of the scale are so small that the niceties cannot be appreciated properly by the naked eye and a catalogue illustration! It is perhaps easier to note some intermediate steps:
wedge
quarter hollow ground
half hollow ground
full hollow ground
Wedges seem to be appreciated most by those with tough, wire-like beards, whilst full hollows are for experienced shavers with less resisting whiskers! Why do I say experienced? Because it takes quite a bit of skill to steer a wafer thin bit of metal over your face without cutting yourself - sometimes the edge appears to be as thin as foil and readily deflects - tough beards can wreak havoc with such blades. For most of us the ideal grind will be found somewhere between the two extremes. I usually advise people to start with stiffer blades than full hollows, such as half hollow and quarter hollow. If this does not suit, then you can move up or down the scale to something more accomodating.
A lot of the non-german vintage razors I see marked "Full Hollow" and "Extra Hollow" are in fact a deal thicker than the textbook full hollow ground, though, and can be an ideal starter razor.
An (abridged) diagram may be seen here
