What is the white smoke behind planes in the sky??

They actually have a name: contrails. It consists of water vapour that is exhausted from the aircraft engines. Since the temperature high in the sky is well below the freezing point, this water vapour freezes immediately to from tiny ice crystals. So what you observe is a nice trail of small ice crystals. In fact, contrails are almost the same as real clouds, only different because they're created by an airplane (and have a different shape). I hope you understand now.