While I do not know for certain, I get the feeling it is sort of.. a craft which mixes magic (Hence the Hex part) and technology (Obviously Tech, ergo; Hextech?).
Seems to be implied by the 'hextech' items, obviously the Hextech Gunblade is a gun, which is technology, though numerous items its made from are quite magicy, and there are the glowing runes along it if I remember correctly, denoting the magic.
All just a presumption on my part. One thing is blatant though, that it is heavily Steampunk influenced, just modified to fit a bit more smoothly into the setting, which is fine by me.