When a ticket is booked through IRCTC or Railway station window counter then a unique 10 digits reference number is generated and against which all the details like passenger names, age, gender, source station, boarding station and destination along with the booking status and current status of the of the passengers are saved in the Indian Railways database and the 10 digit unique reference number is called PNR number which is the short form for Passenger Name Record.
When we book a ticket in a particular train, sometimes we do not get the confirmed ticket and Waiting List (W/L) or RAC booking status ticket is issued instead and these status always change when someone cancel their ticket with Confirmed or RAC booking status. To query and know the new or current status of your booking and commonly known as PNR Status.
PNR number is generally printed on the Top Left corner of your ticket which are booked through the station window and on the E-Tickets which are the tickets booked through IRCTC website and other travel agents are mentioned on the top section along with the other booking details.