Technically, there is a difference between a training log and a training journal.
Information entered in a training log is just basic information about training times, dates, and exercises.
A training journal contains other information, like the way you were feeling that day; the way the dog was feeling; weather conditions, etc.
Personally, I think that keeping a kind of combination of a log and journal is most helpful.
A record or history of your training sessions with your dog will prove to be an invaluable tool in future training sessions.
A good log/journal combination can help you discover where problems in training come from and even how to correct them.
There are many factors that determine the effectiveness of any training session.
During some training sessions, it seems that you can't teach as fast as the dog can learn.
During other training sessions it seems that no matter how many different ways you go about it, you just can't get the idea across to the dog.
Sometimes, dog training success can be measured in leaps and bounds.
But other times, the only way you can actually see any progress is by looking back at where you began.
Log/training journals can provide encouragement for you when it seems that you are making no progress at all.
The most important thing that a log/journal can provide for you, though, is a history of training that you can use to plan future training sessions.
You can log back at the entries in your log/journal and determine which training techniques worked, which training techniques did not work or which training techniques were the most effective.
In short, keeping a good and detailed training log/journal can make you a better and more effective dog trainer.