An internship is an agreement between you and a company or organization for a fixed period of time, such as a semester or quarter. You agree to work for them and they agree to mentor and teach you; internships can offer valuable insight into a particular field or career. Many internships are organized through colleges and high school programs. Many companies go on to hire their successful intern's full time.
A student gets to learn ample amounts from an internship, be it the work, the work culture, the experience, the office atmosphere etc.
Note : Not all internships are paid. Some of them are also unpaid. Always make sure to find out whether an internship is paid or unpaid.