The Bullet Journal is a system devised by Ryder Carroll to take down notes, keep a list of to-dos and organise your calendar in a pen and paper format. I have discovered the Bullet Journal about 2 months ago and I am loving it since. The system is simple to implement and doesn't need you to purchase any particular notebook. All you need is a notebook and a pen to get started.
When I was studying, my school has always stipulate that we write down what our assignments were into a diary, so that we didn't have any reason to say that we didn't know about things. This habit has carried onto my work life. Howver, I realise the calandar system doesn't always work. I have tried one day per page notebooks but some days they are enough but other days they are not. These systems left restrictive and inflexible. Not long after I fell out of the habit.
I start many different notebooks with the intention to keep track what I am supposed to do and when they are due. Yes, I have tried many apps for this purpose too. However, I am a tinkerer, I love trying new apps so I always had trouble sticking to an app or a system.
The Bullet Journal changed that all for me. It is flexible to keep up with my needs, be it a list of to-dos for the day or month, a list of schduled events for the month, or even just a simple list of items that I need to buy. I highly recommend you to check out the website to get a feel of the system.