ERS(Electronic Reservation Slip) -The printout in standard specified Performa containing reservation particulars, and instructions for use which can be used by the Passenger along with the relevant authorized Identification, as travel authority for performing the journey. (See Performa / sample here) link showing ERS. The user of the ERS is governed by these rules and the instructions available on the ERS itself.
Virtual Reservation Message (VRM) - A screen-shot of the e-ticket displayed through laptops/palmtops/ Mobile Phones is referred as Virtual Reservation Message (VRM).