InternshipAn internship is a temporary work experience opportunity at a company or organization that often last for several months. This experience can be very useful in terms of gaining hands on experience in the field that someone ultimately wants to work in. Many employers look favorably on internship experience as it implies you already have an understanding of the work required and already have some of the required skills. Internships can be very useful resume builders in terms of getting a full-time job and many interns find a job at the organization they do an internship at.
Internships can be paid or unpaid depending on the organization, and even many unpaid ones will have some form of compensation like a housing stipend or reimbursements for transportation costs. Most internships are done once someone is already near the end of their education (be it high school or college) as often the expectation is that individuals will already have some of the skill sets required to perform the job.
ExternshipAn externship is a temporary work experience opportunity that is shorter in duration than an internship, ranging from a single day to a week or two. The experience gained is far more limited and is often considered 'job-shadowing' more than work experience, as the term is not really long enough to work on any projects. While an externship is less likely to generate a full-time job it can be a useful tool to securing an internship with an organization.
Externships are almost never paid, as there is not typically very much work actually performed during an externship. The exception can be when professionals are brought in for a small project or to fill a staffing shortage, though this is really more part-time work or 'temping' and not an externship. Most externships are completed early during someone's education as the skill sets required to actually work at the organization aren't really there yet. They function really as an introduction to what the job is and what someone can expect if they were to actually work there (or have an internship) later.