Dear Anshika, Initially all colleges of DU were concentrated in one region in North Delhi and faculties or departments of each course inclusive of sports grounds, hostels and main offices were built nearby. This entire hub of 8-9 colleges and above mentioned buildings got its name north campus. When DU expanded to South Delhi as well near Dhaula Kuan, with 5-6 colleges in an area; it was called the South Campus. 

Gradually more colleges were built in scattered areas of Delhi with no as such university hub, such stand alone colleges are known as the off campus ones.

All the best 

Feel free to ask for any query