tpi means thread per inch Sheldon has a page on tire faqs http://www.sheldonbrown.com/tires.html

I paid extra for the kevlar, but the city streets I ride on mostly are littered with broken things waiting to let the air out of my tires. I only have kevlar on my commuter, my other bikes are without it. No real difference in the amount of flats I get. I always seem to get them at the worst time possible, but is there ever a good time to get one?