Fit is all about the shape of the jacket: slim, regular or loose. The fit you prefer will depend on the style and look you prefer.
Slim fit jackets offer a form fit, tailored more at the shoulders, mid body and waist while still ensuring full comfort. If you want to avoid a baggy or bulky look, slim fit is for you.
Slim/Regular fit jackets are a happy-medium keeping with the trends without being tight. These jackets are slightly tailored and ideal if you want a classic style with plenty of comfort and good mobility.
Regular fit jackets offer a traditional fit, typically shaped to fit just below the waist. Regular fit jackets offer a shaped fit without being tight or constricting, providing a classic style.
Regular/Loose fit jackets will provide you with a classic look but plenty of room for layering. While avoiding the overly-baggy look, this jacket is great if you want more mobility and more room than a regular fit.
Loose jackets will offer more room in the shoulders and chest than a regular fit jacket. If you are looking for a relaxed fit or want a baggier style a loose fit is for you.