Bid'ah

Assalamaliakum. I wanted to ask you "What is bid'ah?" I was told that it is an innovation. I want to know if the actual act is an innovation or the timing of it. for e.g. reading durood shareef loudly after jamah. I believe that reading dur ood is not bid'ah but does reading it in this manner make it bid'ah? Jazakallah

Aslamwaliakum. Please could you tell me whether the practice of doin pray on thursday night (khattam as pakistanis call is) is the Sunnah of the Prophet SAW or innovation. What i mean by pray is that there is food placed on the table every Thursday  night and a dwa is read out. Please could you clarify this with me. Jazakallah and Slamwalaikum