When I was younger, I used to have this pink composition notebook with stickers all over it. It also had suggestions like, "keep out" and "private" written on the cover. It was my very own дневник diary. This word can also be used to mean datebook, but today I am using it as diary.

Here are some example sentences:

Каждый день я пишу в своём дневнике.
Every day I write in my diary.

Маша спрятала дневник под кровать.
Masha hid her diary under the bed.

Мои личные днeвники дома, в коробке.
My personal diaries are at home, in a box.

Анна рассказала мне о дневниках из Франции.
Anna told me about the diaries from France.