Shape
There are two cigar shapes: cylindrical and torpedo. Cylindrical cigars have a constant diameter from head to foot, allowing for a smooth and constant airflow through the cigar. Torpedo cigars are thicker in the middle, resulting in a more-concentrated airflow.
Length
The length of a cigar determines the smoke temperature as well as how long the cigar can be smoked. Longer cigars tend to have cooler smoke and can be smoked for a longer time. Shorter cigars produce a hotter smoke and burn out more quickly.
Diameter
A cigar's diameter is functions in a similar fashion to that of the length. Wider cigars last longer and also give off a fuller, smoother flavor.
Wrapper
A cigar's wrapper is also a determinant in the cigar's flavor. Dark brown or black wrappers indicate the smoke will be rich and full bodied, while lighter wrappers indicate the smoke will be lighter and smoother.
Tobacco
The tobacco used in a cigar is a key determinant in its flavor. As with any other agricultural product, the origin of the plant and conditions under which it is grown contribute to its quality. Arguably, the finest cigars come from Cuba (but are illegal to transport into, or possess, in the U.S., as of 2010). Other cigar-growing regions are mostly in tropical climates such as the Dominican Republic, Honduras, or Costa Rica.