der Winter
winter
The coldest of the four seasons, which follows autumn and comes before spring.
In diesem Winter hat es außergewöhnlich viel geschneit.
This winter, it snowed an unusually large amount.

winter
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winter
The coldest of the four seasons, which follows autumn and comes before spring.
In diesem Winter hat es außergewöhnlich viel geschneit.
This winter, it snowed an unusually large amount.

To indicate a point in time within the season, you use 'im Winter' (in winter). To express a duration over the whole season, you can say 'über den Winter' (over the winter / through the winter).
Im Winter fahren wir oft Ski.
In winter, we often go skiing.
Other useful words are derived from the noun 'der Winter' (winter), such as the adjective 'winterlich' (wintry, e.g. winterliches Wetter — wintry weather) and the verb 'überwintern' (to spend the winter / overwinter).
Die Landschaft sah heute Morgen sehr winterlich aus.
The landscape looked very wintry this morning.
Common combinations are 'ein kalter/strenger Winter' (a cold/harsh winter) or 'ein milder Winter' (a mild winter). When the cold season begins, people often say 'der Winter kommt' (winter is coming) or 'der Winter bricht herein' (winter sets in).
Wir hatten dieses Jahr einen sehr milden Winter.
We had a very mild winter this year.
You can recognize the figurative meaning from the context. When 'Winter' (winter) is connected with emotional or abstract terms like 'politisch' (political), 'wirtschaftlich' (economic), or 'Winter des Lebens' (winter of life), it does not refer to the season.
Nach der Firmenpleite begann für die Mitarbeiter ein wirtschaftlicher Winter.
After the company went bankrupt, an economic winter began for the employees.
summer
'der Sommer' and 'der Winter' are two opposite seasons. Summer is the warmest time of the year and comes between spring and autumn. Winter is the coldest time and comes after autumn and before spring. The contrast is especially important for weather, clothing, travel, and leisure activities: in summer, people often think of warmth and long days, while in winter they tend to think of cold, snow, or short days.

Im Winter trägt sie morgens einen dicken Mantel.
In winter, she wears a thick coat in the morning.
Im Sommer fahren viele Familien ans Meer.
In summer, many families go to the seaside.