An échappé is the opening of both feet from a closed to an open position. There are two kinds of échappés. Echappé sauté, which is done with a spring from the fifth position and finishes in a demi-plié in second position, and échappé sur les pointes, or demi-pointe, which is done with a relevé and has straight knees when in second position. In each case échappés are done to the second or fourth position, both feet traveling an equal distance from the center of gravity.
An assemble is when you throw one leg up and springs off the other, while ascending the raised leg continues to rise, landing on both feet closed together back in fifth. Assemble can travel in the direction in which the leg was raised, and can be performed while turning, or with a beat.
Here is an attached video of how to do it.
Here is an attached video of how to do it.