Immigration visas are legal documents that allow foreign nationals to enter and stay in the United States for a specific purpose and period of time. Some of the most common types of visas include: 

F1 Visa: 

The F1 visa is a nonimmigrant student visa that allows foreign students to enter the United States for academic studies at an accredited institution. To be eligible for an F1 visa, the student must have been accepted into a U.S. college or university, be enrolled in a full-time academic program, and demonstrate sufficient financial resources to support themselves during their studies. F1 visa holders are allowed to work on-campus for up to 20 hours per week during the academic year and full-time during school breaks. After graduation, F1 visa holders may be eligible for Optional Practical Training (OPT), which allows them to work in the U.S. for up to 12 months (or up to 36 months for STEM fields) in a job related to their field of study. 

H1B Visa: 

The H1B visa is a nonimmigrant visa that allows U.S. employers to hire foreign workers in specialty occupations that require at least a bachelor's degree or equivalent. To be eligible for an H1B visa, the foreign worker must have a job offer from a U.S. employer, possess the required skills and education for the job, and the employer must file a petition with the U.S. Citizenship and Immigration Services (USCIS) on the worker's behalf. H1B visa holders are allowed to work in the U.S. for up to three years, with the possibility of extending their stay for up to six years. 

OPT: 

Optional Practical Training (OPT) is a period of temporary employment that allows F1 visa holders to work in the U.S. for up to 12 months for non-STEM fields, or up to 36 months for STEM fields in a job related to their field of study. To be eligible for OPT, the F1 student must have been enrolled in a full-time academic program for at least one academic year and must apply for and receive an Employment Authorization Document (EAD) from the USCIS.

OPT stands for Optional Practical Training, which is a period of temporary employment that allows F1 visa students to gain practical experience in their field of study after completing their academic program. OPT is typically used for internships, cooperative education programs, or other work-study programs that are directly related to the student's academic program.

To be eligible for OPT, F1 students must have been enrolled in a full-time academic program for at least one academic year and must apply for and receive an Employment Authorization Document (EAD) from the U.S. Citizenship and Immigration Services (USCIS). The EAD serves as a work permit and authorizes the student to work in the U.S. for up to 12 months (or up to 36 months for STEM fields) in a job related to their field of study.

During the OPT period, students are expected to work in a job related to their field of study and to report any changes in employment status to their Designated School Official (DSO). OPT employment can be paid or unpaid and can be full-time or part-time, but the student must work at least 20 hours per week to maintain their legal status.

It's important to note that students who engage in unauthorized employment or violate the rules and regulations governing OPT may jeopardize their immigration status and may face deportation or other immigration consequences. Therefore, it's crucial for F1 students to understand the requirements and restrictions of OPT and to work closely with their DSO to ensure that they are in compliance with all regulations.