In SHARI'AT a person who intends to TRAVEL a distance of 77 kms (48 miles) or more, is called a MUSAAFIR.
A person who travels 77 kms or more and intends to REMAIN at one's destination for LESS than 15 days, is also a MUSAAFIR.
A Musaafir who intends remaining at his destination for 15 days or more will only be a Musaafir during his journey. Once he reaches his destination, he will not be a Musaafir.
A Musaafir should make QASR of the ZOHAR, ASR and ESHA (FARZ only), i.e. one must perform TWO Farz only instead of FOUR rak'aats.
There is no Qasr in the Farz of Fajr and Maghrib. Similarly there is no Qasr of Witr, Sunnat or Nafl Salaat.
A Musaafir who performs his Namaaz behind a MUQEEM IMAAM (who is not a musaafir) should perform the full four rakaats in the Zohar ,Asr and Esha Farz.
If the IMAAM is a musaafir and muktadi a muqeem, the Musaafir Imaam should complete his Namaaz after two rakaats and there after he must ask the muqeem muktadis to complete their Namaaz by saying: "Complete your Namaaz, I am a Musaafir" The muqeem muktadis should then stand up and complete the remaining two rakaats without reciting Surah Faatiha or any other.