CPT

The OPT allows you to gain work experience after your studies. It is usually awarded for 12 months with a possible extension upto 24 months for STEM graduates.

The CPT is a work authorization, granted to students, during the course of their studies. The CPT can be both part time and full time, with average duration being around 20 work hours per week. 

CPT as it suggests Curricular Practical Training is a training that applicants can take up while they are enrolled in a full-time course with an F1 visa. It can be either a part-time on or off campus job, research opportunities, etc. On the other hand, OPT is Optional Practical Training can be either taken up during or after graduation. Those studying STEM courses, can also go ahead with STEM OPT Extension. 

CPT full form is Curricular Practical Training. Whereas, OPT stands for Optional Practical Training. You can apply for both of these simultaneously. However, it should be noted that the CPT can be authorized only until the last day of the final quarter.