die Postkarte
postcard
A card made of cardboard, usually with a picture on the front, that is sent by mail without an envelope to convey a short message.
Ich habe dir aus dem Urlaub eine Postkarte geschrieben.
I wrote you a postcard from vacation.

postcard
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postcard
A card made of cardboard, usually with a picture on the front, that is sent by mail without an envelope to convey a short message.
Ich habe dir aus dem Urlaub eine Postkarte geschrieben.
I wrote you a postcard from vacation.

The word is easy to understand if you break it down into its parts: 'die Post' (the mail/post office) and 'die Karte' (the card). So it is literally a card that you send by mail.
Früher hat man viel öfter Postkarten verschickt als heute.
In the past, people sent postcards much more often than they do today.
The most common verbs in connection with 'Postkarte' (postcard) are 'schreiben' (to write), 'schicken' (to send), 'bekommen' (to get), and 'erhalten' (to receive). The context 'aus dem Urlaub' (from vacation) is also often mentioned.
Hast du die Postkarte bekommen, die ich dir aus Spanien geschickt habe?
Did you get the postcard that I sent you from Spain?
A typical sentence follows this pattern: subject + verb + dative object (recipient) + accusative object ('die Postkarte'). You send 'jemandem' (to someone, dative) 'eine Postkarte' (a postcard, accusative).
Meine Oma schickt mir jedes Jahr zum Geburtstag eine Postkarte.
My grandma sends me a postcard every year for my birthday.
card / map
'die Karte' and 'die Postkarte' can both be a small piece of cardboard. But 'Karte' is much more general: it can be a playing card, credit card, ticket, or also a card with a message. 'Postkarte' is more specific and means a card that you send by mail without an envelope, often with a picture on the front. Learners may confuse the words because a postcard is a type of Karte. When the topic is sending something with a short message, 'Postkarte' is the precise choice.

Aus dem Urlaub schreibe ich meiner Oma eine Postkarte.
I’m writing my grandma a postcard from vacation.
Zum Geburtstag bekam sie eine schöne Karte von ihren Kollegen.
For her birthday, she received a beautiful card from her colleagues.