Climate in Europe is very multiform. There is no dramatical changes and climate is mainly mild and moderate. There is still different zones in different parts of Europe.

Southern parts of Europe are subtropical, which means hot and dry summers and mild winters usually without snow. Mainly Europe is part of temperate climate. Where weather is pertty mild. Not that hot summers but not that cold winters either. Northern Europe has Polar climate which means cold winters and mild summers.

Europe is very safe travel destination, because it is very rare to have dangerous weather variations or big storms. Of course now when climate is changing storms and floods are more normal than they use to were.

google.com

Because of nice and mild weather even in winter season Southern Europe attracts tourists also in winter season. Specially Spanish coast is very popular to older people to go spent their retirement days. But because weather stays pretty warm all year around there is lots of activies to do even in Winter season. Different kinds of sports attract people to travel e.g. Golf.

Summer season is high season for Southern European countries. Weather gets really hot and dry and beaches attract lots of people from Europe and even outside Europe.

Malaga, Spain picture from google.com

Northern Europe and Alps are complete opposites of Southern parts. Cold winters guarantee that there will be snow. These countries attract lots of tourists in Winter season. And it is their main tourism season. Although nowadays people and tourists are more interested to travel these areas also in other seasons like Summer.

Pyhätunturi, Finland Picture from google.com

Specially Scandinavian countries are advertising other seasons too more than they use to did. www.visitfinland.com advertises all seasons with their special characteristic features.
Video below is from Finland, it promotes magical summer nights and nature. Like in this case traditional Winter destination tries to transform more diverse tourism destination. Because Northern parts of Europe has also very beautiful and different landscape, they have good trump card, when they can promote more exotic ways to travel. e.g. hiking in nature.