There are two types of internships available. Curricular Practical Training(CPT) is usually done as part of a class during the semester or as an internship during the summer. The training has to be "part of your curriculum" so it is recommended you complete it for credit. For more information on the limitations of this internship and how to apply, click on the link below.
The second type is usually done after graduation. It is called an Optional Practical Training (OPT) and is part of the F-1 visa regulations. For more information on deadlines, cost of the application and procedures, click on the link below