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. In reality the stars in those constellations have no real connection to each other. The can be vast distances from each other, but looking at them two-dimensionally they seem to form patterns that are easy for us to identify.
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. We are all bound by gravity. Our solar system and the many of the stars that we see are on the edge of the Milky Way. If you are in an area that is free from light pollution, on a clear night you can see a long dusty looking stream in the sky that is part of the more populated part of the Milky Way. We can see other galaxies, but they look just like dusty objects in the sky. There are billions of galaxies out there, some of which we can see with the naked eye or a telescope. The Andromeda galaxy is the most distant thing we can see with the naked eye, at about 2.5 million miles from us.