When you book a bus, you are given a reference no.
When you transact in a bank, you are given a transaction/reference no.
Similarly when you purchase a ticket in Indian Railways, you are given a PNR No. which stands for Passenger Name Record.
Not...
more...

everytime you buy the ticket, you are given a confirmed ticket. (Depends on availability)
When you buy a Waitlisted or a RAC Ticket (assuming you know what these two stands for else read Steven Sir's article from Indiamike; link given at the last of admin's blogs), you are totally unsure whether the ticket would confirm, move to rac, or stay back in WL (Wait list).
######3
This is where the PNR PRediction comes into picture where experts/experienced people predict about the future status of the PNR from their own experience/instinct/guess.
This provides a base for the traveller to hold back to the ticket or cancel it. The last and final call always rests upon the traveller. He is just given a prediction and it is upto his discrepancy whether to stick to the ticket, cancel it, or go for a fresh one.