A New Year's postcard used to write New Year's greetings; generally synonymous with 'nengajo'.
年賀葉書を買って、友達にメッセージを書いた。
I bought New Year's postcards and wrote messages to my friends.
年賀葉書は1月1日に届くように配達される。
New Year's postcards are delivered so that they arrive on January 1st.
今年は年賀葉書ではなく、メールで新年の挨拶を送った。
This year I sent New Year's greetings by email instead of postcards.