It’s a indian government site which provides online course by iit’s faculties. You can get all courses of nptel on swayam app. I learnt English for communication course by this website. You can’t register for any course in any time . Registration form of any course valid for certain limit of time and when course is over then you must need to give exam if you want to get certificate and for certificate you must need to pay one thousand rupees. The way of explaination of theory is best . You can apply for most of the course in December - January and June - July. You can’t get unlimited access of any course and you must need to finish course in certain limit of time.