PNR number is a record locator that is created and stored in the computer reservation database. It acts as the primary reference that helps in finding the updated reservation status and confirmation of ticket. Nowadays, Indian railways has offered a lot of facilities for checking the PNR of the ticket by mobile, SMS, telephone and over the internet.
If you are using the IRCTC website for checking the PNR status then you should note down that the online PNR Status Enquiry is available only during 0500 Hrs to 2300 Hrs IST. For faster processing of the passenger enquiries, Indian Railways has decided that PNR Status is only available for those PNR's where the date of journey is within 10 days from the current day. One has to offer the details of journey date in addition to PNR number for obtaining the current status.
Indian Railways has made the lives of the passengers very convenient by offering the Train PNR Status on the mobile phones. It has markedly increase the relief and comfort level of the passengers.
In order to get the PNR Status via mobile phone, one is required to type in 10 digit PNR numberthat is printed on the ticket and sends it by SMS to 9773300000. Indian Railways is offering this facility in collaboration with Google. Passengers are only required to message their PNR number to 9773300000 through their cell phone for checking the current status of PNR. One should take care not to prefix 0 or +91 for sending SMS. Passengers will be charged at the price of standard SMS only and there are no premium SMS charges applied.source : http://pnrstatus.co.in/irctc-pnr-status.html