It is threads per inch as has been said. You may also see it called EPI, ends per inch. Euro-metric madness being what it is it is odd that it has not been converted to metric and inverted from the usual sense, micro-meters per thread or something like that. I guess I should not complain, the European bicycling industry is still charmingly unit neutral to a large extent.

Anyway a lot of the loss in a tire is due to sidewall flex. There are two ways to reduce this. One is by high inflation pressure which reduces the flex and the other is to change the tire construction to reduce the energy lost when the tire does inevitably flex. Supposedly high thread count casings have less energy loss and that is why you tend to see them mostly on high end racing tires in the narrower widths where the thread count may be 320 tpi or higher. So, in theory the 180 tpi tire will be faster than the 60, and 100-180 is as high as I have found in city/touring tires with good puncture resistance. TPI is an absolute measure that can be compared from make to make (although there is no way to know for certain that one brand's 120 TPI tire is as efficient as another's). Everyone rates their puncture resistance on their own internal scale and I know of no way to compare them. All you can do is ask people who own a given tire how it has worked for them with regards to punctures. Of course if you stick with one brand then I would imagine that the brand rating system is accurate, the tires they claim are puncture resistant should be better than their unprotected tires.

Ken