A teaching journal is a written record of your teaching experiences, usually committed to paper (or disk) while the experiences are still fresh. Keeping notes on your own teaching can be useful to you, much like keeping research notes. The simple act of writing an experience down will help you retain it, and will almost certainly help you reflect on it and learn from it. Re-reading the material later on allows you to look back on your experiences with critical detachment. This is especially useful if the experience was unpleasant - e.g. if section did not go well. In this case writing about it, and then reviewing it later, can help you distance yourself from the experience, so you can analyze what went wrong, and avoid the same mistake in the future.
A useful way to keep a teaching journal is to simply incorporate it into the same document in which you plan out your presentations. Specify, before class, what you plan to cover and how. Then, after class, you can note down quickly: