1)  Take a look "2012-11-28 15:24:38 UTC" - was first halving  50 -> 25

it was block #210,000 http://blockexplorer.com/block/000000000000048b95347e83192f69cf0366076336c639f9b7228e9ba171342e

210,000 * 50 = 10,500,000 BTC was minted at this time. ( Half )

2) Next one will playing with quarter. Similarly. 

   * 25 -> 12.5
   * There will be minted three quarters (3/4) of all possible BTC.