"PIREPs are filed at unscheduled times with stations having sending capability to WMSCR [Weather Message Switching Center Replacement] for dissemination on the Service A domestic aviation weather system. These reports must be entered into the operational system as individual reports, not appended to a surface observation."
International standards may vary, but at a minimum the PIREP would contain an identifying header, the location of the aircraft, the time, and flight level. A PIREP will also include UA or UUA used to designating the PIREP as either routine or urgent. But all these data points are also formatted. The location has to use a 3-letter NAVAID ident code only, Time is given in 4-digit GMT, etc. So an Acual transmitted PIREP may read "UA /OV RFD 170030/TM 1315/FL160/TP PA60 /SK 025 OVC 095/180 OVC
/TA -21/WV 270048", which while unintelligible to us, can be received by a computer and later translated back into readable English.
An AIREP (Air Report) is very similar to a PIREP. The P in PIREP stands for Pilot not Position. The AIREP actually covers position data. Obviously they are of little use while in range of radar control. They cover: aircraft identification, position, time and level. More here. The FAA uses the following description:
AIREPs are messages from an aircraft to a ground station. AIREPs are normally comprised of the aircraft's position, time, flight level, ETA over its next reporting point, destination ETA, fuel remaining, and meteorological information.