A constellation is a group of stars that appear to make a pattern in the sky. When we look at them they look close together but in reality they are usually very far apart and not grouped together in space.
An asterism is a pattern either in a constellation such as 'The Plough' in Ursa Major or made up of brighter stars connecting constellations such as 'The Summer Triangle'..
At different times of the year we can see different constellations due to the rotation of the earth and the tilt of our axis and so some are seasonal. Some stars appear throughout the year from our position and they are called circumpolar.