I started writing in a diary when I was 17 or so years old. It started as a text file on my computer. Then I recently put it on Google so if the house burns down it won't be lost forever. It's on a Google server somewhere where it will remain forever. Unless someone cracks my account and deletes it, that is.
In my diary basically I write about my relationships, family, what the hell is wrong with me, depression, ideas and insights, etc. Just thoughts and things that come up that I think is worth writing down. I don't write every day, but I usually write something once every week or two. It helps put things into perspective. It's also interesting to read the things you wrote six years ago. I wonder what I'm going to think about what I'm writing now 20 years into the future! Scary thought.