The primary difference between an internship and an externship is that an internship is a more involved work experience within a company, and an externship is a less formal job shadowing experience. An internship lasts longer, often several months, while an externship often lasts less than a week.
With an internship, a student is sometimes paid and often receives college credit for the hands-on experience. With an externship, the person is normally not paid and doesn't receive college credit. The goal of an internship is an in-depth experience in a career field. An externship allows a person to shadow a professional in his job role to gain insights.