I've been keeping a journal for years. Far from the "dear diary" that's often shown in TV shows or movies, it's more of a way for me to remember things later. It's often helpful to go back and remember WHY you bought that certain car, WHY you took a certain job, and what the thought process was at the time. Especially if those conditions change later and you find yourself saying, "How did I get here?". A journal can help with these things, as well as provide a great record of your life. Few of us will ever sit down and actually write our autobiography, but a good journal can fill the role quite nicely.
Essentially, a journal can be any form you want. I know people who have monthly journals with one page per month, and they jot down things for the entire month on that page. I also know people who devote 2 full pages for each day, because they have lots of details and meetings to record. A journal entry can be very brief if you want it to, such as, "Late for work due to traffic. Lunch with the guys at Joe's Pizza, met the family for dinner in town." Nothing fancy, but it gets the point across.
But then there are times when you really want to record more details, and that's where I'm at now. Truthfully it's where I've been many times, I'm just making a return trip. For many years I've double-journaled. I've kept a paper journal with my meetings jotted down in it, plus a terse recording of what happened during the day, both business and personal. I also kept more fully-formed thoughts in a digital format, where space didn't matter. I could also include digital media, of which we have so much in our lives: camera phone pictures, video footage, links to interesting web pages, etc.
Of course it was double-duty to maintain both of these, but I did it for years. The separation of terse and verbose postings seemed to provide a nice separation for me, a natural delineation. However last year I stopped, opting to save some time by maintaining only the written journal.
I can't say it's been a perfect success.
One drawback is the space issue. It's a physical medium, so I have to journal in the space provided. I'm using the week-on-2-pages format. Truthfully, sometimes this is the perfect size, and other times I want more space to record details. This is especially true on the weekend days, where they limit it to half the space they give for weekdays. Don't they think lots of stuff happens during the weekend?
The other drawback is the inability to include things like pictures and such. While this is inherit in an old-school paper approach, it's still worth noting. I find there's many items I could include if things were digital.
So that leaves me looking around for possible digital solutions, and re-thinking my approach. In the past I've spun my own HTML pages using CSS and used that as a template for a digital journal. That worked well and I could tailor it exactly as I wanted, but dealing with HTML tags and some formatting issues often took time, and the process seemed to be as much about my template as the writing itself. Then years ago I caught wind of TiddlyWiki and loved it, ended up using it for years. Learning the wiki markup syntax was easy, fun and suited journal writing pretty well. The drawback was TiddlyWiki does some funky things if you journal out of order (say, you put in an entry today for 10 days ago) in that it makes those entries look like today's entries. There was a workaround, which is to modify the time-stamp in the source, but it was still a hassle.
I'm currently looking for something that supports fast writing, doesn't get in my way, yet still outputs nicely formatted material. Markdown syntax looks like the clear winner, so I'm thinking about it.