Constellation is simply a group of stars. They sometimes form patterns that are named after what shape they form. At many places, they have religious beliefs associated with them. The shapes of constellations continuously change due to relative motion of earth. As earth revolves around the sun, sunlight causes some areas of sky to be visible during different seasons. The constellations are differently seen in Northern and Southern hemispheres.
There are in total 88 constellations. Some examples are Ursa Major, Orion, Pegasus etc.