What is a STANDING WAVE? How is an echo formed?

Standing wave is phenomenon caused by waves interference. A bunch of waves summed together results on a wave that appears to be static.

Echo is the wave reflected due to obstacles on their way. For sound we are only able to notice a echo when the reflected wave takes more than 0.7s to come back, otherwise (t <= 0.7s) we call this effect as reverberation.