An internship is a short-term job that allows the worker, or intern, to gain introductory experience in a certain profession. 

• As an intern, you will work for a company or organization and gain hands-on and practical experience you can use for future jobs.

There can be a lot of variation between internships. 

• They may be part-time or full-time, paid or unpaid, take place over the summer or during the school year, and last anywhere from a few weeks to a year or more. 

• Because internships are designed for people with little or no experience in a particular career field, most interns are students or recent college graduates. 

Source: Prepscholar.com