Even Allah and the angels engage in salaat on the Prophet(Pbuh)-Says Al-Quran
Allah Ta'Aala commands in the Quran Sareef: O people of Iman! Send salaat (durood) and salaam (salutation) in abundance on him (i.e. rasulullah sallallahu alaihi wasallam). According to the Quran Majid even Allah Ta'Aala and the Malaa-ikah engage in salaat (durood) on the Nabi sallallahu alaihi wasallam. In Arabic the term salaat has several meanings, rahmat (mercy), Dua (supplication), and madah (praise).
When the word is related to Allah Ta'Aala it means the bestowal of rahmat. The salaat of the Malaa-ikah means there Dua for rasulullah sallallahu alaihi wasallam. When salaat is recited on rasulullah sallallahu alaihi wasallam by the mumineen it means the invocation of praises on rasulullah sallallahu alaihi wasallam.
The word salaam means to be kept safe from defects and calamities. so as salaam alaikum means May you be kept safe from all defects and calamities. The word salaat is popularly translated as durood