There are several general types of polymer used to manufacture most high-performance caulks: silicones, polyurethanes, latex-acrylics, and solvent-based “block copolymer” synthetic rubbers (which include butyl as well as some of the newer proprietary synthetic formulations).
The best type of caulk for the job depends largely on the materials to which the caulk must bond. For bonding dissimilar materials (for example, wood to masonry, look for sealant that can handle a lot of joint movement.
For the most part, polyurethane caulks provide the best all-around performance for exterior applications, but they are more expensive than most other caulks and can be difficult to clean up (below).
For more information, see the JLC Field Guide.