Derek Rose underwear covers three main uses. Boxer shorts have the loosest fit, trunks sit closer to the body with a shorter leg, and undershirts are designed to layer beneath shirts or knitwear.
Fabric changes the feel as much as the shape: woven cotton is crisp and relaxed, stretch jersey suits closer-fitting trunks, and soft lightweight fabrics work well for undershirts.
Start with the fit and type of underwear you prefer, then look at the fabric and neckline where relevant.