Note: Jack Daniel's Tennessee Whiskey on this site refers to the black label Old No 7 brand.
Jack Daniel's Tennessee Whiskey is a sour mash whiskey produced in Lynchburg, Tennessee. It is basically a straight bourbon but Jack Daniel's eschews this classification and calls their product Tennessee whiskey. The one major difference between Tennessee whiskey and bourbon is that Tennessee whiskey is charcoal filtered whereas bourbon is not.
Because of the charcoal filtering, Jack Daniel's is a very smooth spirit. I find that some of the sharp edges that get removed from filtering give American whiskies character and distinctiveness so I tend to drink bourbons and ryes.