Well, Do you want to give the Exam or Do you want to Know the Technical Systems involved .
Online Exams are Exams which are conducted on PC. Some need an Internet Connection where for Some Exams you need to be in a Centre where there are 100s of Computers
Technical Points for a Centre – A Few 100 Computers connected to the Main Server and the necessary Software provided by the Online Testing Company ( Every Testing Agency have their own Software for conducting Tests).
In India , Some Tests linke BITSAT, Manipal Admission Test (from 2009) and CAT is also going Online