postcard vs postal card

postcard and postal card both are nouns.

Word	Noun	Adjective	Verb	Adverb
postcard	Yes	No	No	No
postal card	Yes	No	No	No

As nouns, postcard and postal card are synonyms defined as:

postcard and postal card: a card for sending messages by post without an envelope

Other synonyms of postcard include mailing-card, post card.

postcard (noun)	postal card (noun)
a card for sending messages by post without an envelope	a card for sending messages by post without an envelope