Yeast leavens bread by providing CO2 gas which is a by-product of the fermentation stage.
Baking powder also provides CO2, but through a straight chemical reaction rather than a biological one. All baking powder you're likely to find at the store is "double acting" which simply means you get two rises, which doubles your chances of getting a lighter bread. This action is produces by the chemical reaction of a base (the baking powder) and an acid (usually cultured buttermilk for biscuits). The first rise happens at lower temperatures (room temperature) and the second happens at higher temperatures (oven temps). So for baking powder to work well (assuming it's not gone bad over time) you must have acid (buttermilk) and the base (baking powder). Some recipes also call for baking soda. Just make sure you measure out everything to get the right proportions and don't substitute something like regular milk for the buttermilk.
The next thing that helps make biscuits lighter is the fat content of the dough. The fat should be blended into the flour such that small globs of it are contained throughout the dough. When the fat melts, the CO2 from the leavening agent fills the spaces and causes the biscuits to rise. So the fat, base, and acid all have to work together to make the magic happen. Otherwise you get hockey pucks.
Here are a few tips to insure success. As others have said, make sure your baking powder is good as it does expire and won't be as effective. Make sure the fat doesn't melt before baking. I prefer to use shortening rather than lard because it has a higher melting point and is easier to work. I like to use butter also because it has a different melting point and ads taste, but I use about a 1:3 butter/shortening ratio. You still have to be careful. Don't make biscuits in a hot kitchen. Cut the fat into small cubes and chill in the refrigerator for an hour before working. Work the shortening in with your fingers, but be careful not to work it too much. Do not overmix the dough. Add the liquid ingredients and work it until it just comes together. Working the dough too much impedes rising and makes tougher biscuits. After you cut your first biscuits, you can roll the remaining dough out and cut new biscuits, but they won't be as light as the first ones.
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