Constellations are patterns of stars in the sky. Over many thousands of years, human beings living on the Earth have looked up and seen the shapes of people, animals and everyday objects - they 'joined the dots' of the stars to form patterns in the sky. They gave these shapes names, some of which we still use today. But why did they bother with constellations at all? One reason is that having constellations makes it easier to find your way around the sky. This is useful for finding north, or working out the date or the time. When there were no compasses, clocks or watches, this was all very important.
There is another reason why constellations were so important. Imagine going back to a time before paper and pencil had been invented. Life was sparse. If you wanted to draw a picture, you had to scratch it out on a dark cave wall or write it in sand that could blow away. But at night an amazing pattern of bright specks of light would appear above your head. By joining the dots you could see almost any picture you wanted. You could imagine seeing your favourite shapes in the sky. You might tell others about the constellations you have made up. They might tell others, and your constellation could be passed down from generation to generation for hundreds or even thousands of years. This is how many of the constellations got the names we use.
You don't see exactly the same part of the sky every night, though between one night and the next you won't see much of a difference. Over a few weeks you'll definitely notice that you can see some constellations that you couldn't see before and some constellations that you could see aren't there any more. This is because the Earth is moving around the Sun. As the Earth moves round in its orbit, the night side of the Earth (the side facing away from the Sun) faces out to different parts of space, where there are different constellations.
This means that you see different constellations in different seasons. Orion and Taurus are (Northern Hemisphere) winter constellations, because you can see them on winter evenings. Cygnus and Scorpius are (Northern Hemisphere) summer constellations, because you can see them on summer evenings.
So who made up the constellations and their names? The earliest people on the Earth were hunters and gatherers. They looked up into the sky and saw shapes that were important to them - like Orion the Hunter. Much later in human history, English farmers looked up into the sky and saw the shape of a Plough. Russian peasants, looking at the same group of stars, called it Ursa Major or The Great Bear. People in France called it Le Casserole meaning the Saucepan. People in the USA called it The Big Dipper meaning a soup ladle. All of these different names are used today. The only people who have decided on one set of names for the constellations are the astronomers. For example, they always call the Plough 'Ursa Major', and never any of the other names. They had to do this so that they all knew what other astronomers across the world were talking about.
The stars in a constellation have nothing to do with each other; they can be very, very far apart, even if they appear to be right next to each other in the sky. Imagine looking up while standing in a street. You might see your hand next to a street light, which is next to the Moon, which is next to a planet, which is next to a star. All of these things are far away from each other, yet they can be next to each other when you look at them.
If you like, you can look up into the sky, join the dots and make up your own constellations. Tell other people about your constellations. Maybe one day people all around the world will be using one of your constellation names!