Theory:

A diary is a book where everyone can write anything according to their personal thoughts. It may be about their future, or the characters that they care about or anything. 
 
The diary is a safe place where the person can express their feelings, thoughts, opinions about anything they care about. At a point, the writings can reflect the dreams and thoughts of the person to drive an upliftment of life.
Types of Diary:
 
Travel Diary: Travel Diary, or also known as Travelogue, are records written about the travel experiences.
 
Dream Diary: A dream diary records a person's dreams on a regular basis and tracks their themes and patterns over time.
 
Work Diary: A work diary records down the person's reflections about their professional life: everyone can write about their experiences, goals and milestones!
 
Academic Diary: Academic diary is a record of ideas, thoughts and responsibilities for students to manage their time in school better.
 
Fitness Diary: A fitness diary, Workout or Exercise Diary, keeps track of all the records, goals, workouts of a person and even the daily nutrition.
 
Creative Writing Diary: A creative writing diary records poems, stories, songs and other artistic records that have crossed the mind.
 
Secret Diary: Secret diaries are for people to share and write down the most private thoughts of their life.
About Diary Entry
 
A diary entry is a piece of writing that has to be organized by date. The entries in the diary can be the personal opinions, feelings, or wishes about their life. The entries can be short or long.
 
Writing a diary helps to focus on the writing without worrying about the audience. Also, writing a diary regularly helps to improve the thinking processes and can even help to become more creative.
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Diary
 
The sample of diary entry or writing a diary is shown below,
Example:
I woke up at 6 a.m. I had my breakfast and dressed up.
Today, myself and my family went to Fantasize Amusement park near Pune. We started from home at 7 o'clock.