BBT = Basal Body Temperature
it is the temperature taken in the morning first thing you wake up and it is suppose to be taken daily at the same time. then you can do a chart using the temperature... you should start taking the temperature from the 1st day of your period (i.e. the 1st day you see a blood flow which needs a pad) to the 1st day of your next period.
BBT can help to track when you ovulate (O) and thus detemines your most fertile days... which is 2 days before O and O day.
I use this to track my temperature:
Ovulation Calendar and Ovulation Chart - FertilityFriend.com
there is a special BBT Thermometer for this, which have 2 decimal points compared to the normal thermometer... BBT thermometer have a higher accuracy bcos of the 2 decimal place as compared to the normal ones... you can get the BBT thermometer at watsons or guardian at around $10-$20.... i'm using omron brand from guardian... dont get the guardian house brand one... i try that one before.. and it got spoilt after 2 use...
you can read more about BBT here:
Basal body temperature - Wikipedia, the free encyclopedia
there are also alot of information on Ovulation Calendar and Ovulation Chart - FertilityFriend.com