For those unaware, I suffered from a six-month long illness over a year ago, called labrynthitis. It's a viral infection of the deep inner ear, which affects your balance. I couldn't walk or stand very well for months, and found myself nearly passing out or falling over if I stood too long. Let's just say riding in cars, on trains, or in planes was a horrible experience for me - one I had never experienced before as I generally don't have problems with motion sickness. It took months to get a diagnosis, and just as long to figure out what type of motion sickness medicine was the best to use and the most effective (I received varying types from doctors here and from my doctor in the US as well). So for those who do suffer regularly, I now understand your pain.
You can easily get over-the-counter motion sickness medication in Japan, and if you want something even stronger I would suggest seeing a doctor. As for what different types of drugs do, try doing a google search about motion sickness medication to learn more about the different types of drugs and why they are used - that way you can choose something you want or something similar to what you've used in the past.