Here is the extreme basic rundown on dry vs. liquid.
Dry or "active" yeast is yeast put in a kind of suspended animation. The manufacturer causes the yeast to store up a bunch of "food" reserves. Most dry yeast packs come in 11-11.5 gram packs, making approximately 220-230 billion cells.
There are more viable cells in a pack of dry than liquid. Liquid yeast however do not typically have a high enough cell count to innoculate a 5 gallon batch. Wyeast say their smack packs have 100 billion cells. According to Mr. Malty
proper pitching rate for a 5 gallon batch is closer to 180 billion cells.