T8 vs. T-12

Item Subject: Text_1
     >Hooray!!! Someone else who's dicovered T-8's.  Remember that the 
     watts-per-gallon rules are _very_ rough.  T-12's vary considerably 
     from one bulb to another.  When I do estimates on what I'll need using 
     T-8's, I figure as if they were 40W bulbs. (I know they're brighter)  
     It's amazing to me how many people are still paying the costs (up 
     front, replacement and electrical) for T-12's!<
     Excuse my naivete, but what is a T-8 and what is a T-12? Do you need 
     special fixtures for T-8? Sounds like I should only get T-8 bulbs. How 
     do I know this when I'm buying them in the store?