.A constellation is a pattern of several stars that make a picture. A galaxy is a huge mass of billions of stars, dust and gas that is gravitationally bound, moving through the universe.
A constellation is just a pattern of stars in the sky as seen from Earth. There are 88 designated constellations. It is a simple way of us mapping the sky.
A galaxy is a massive group of stars and other elements, such as gases and dust, that are part of one system. Think of our solar system, but on a much more massive scale. Having said that all the stars in a constellation are far from each other, the stars we see are all part of the one galaxy, the Milky Way