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.
First, motion sickness in Japanese is: 乗り物酔い (のりものよい, norimonoyoi)
Probably similar to your home country, motion sickness medication in Japan uses different types of active ingredients. I'm no medical expert, so I can't tell you which type is best, but I can tell you how to find out what active ingredients you are likely to find.
The most common over-the-counter motion sickness medication in Japan typically contains one of the following active ingredients, if not a combination or a small dosage of another active ingredient.
Meclizine 塩酸メクリジン meclizine hydrochloride
Diphenhydramine ジフェンヒドラミン塩酸塩 diphenhydramine hydrochloride
Chlorpheniramine d-クロルフェニラミンマレイン酸塩 d-chlorpheniramine maleate
Pheniramine マレイン酸フェニラミン pheniramine maleate
I'm not entirely clear on all usages and availability of these antihistamines amongst all countries, though I know Meclizine and Diphenhydramine are quite common in motion sickness medicine in the US, while Promethazine is not an over-the-counter drug in the US at all. What about those of you in the UK, Australia, New Zealand, etc.? What are the most common types available in your country?
Supposedly マレイン酸フェニラミン and プロメタジン have pretty strong sedative effects (more so than some of the other antihistamines, although most antihistamines knock me out), but again, I'm not an expert, so please check into these different drugs before you purchase something.
(As far as I know, diphenidol isn't used in the US or Canada)
Scopolamine スコポラミン臭化水素酸塩水和物 scopolamine hydrobromide hydrate
Many types seem to contain a small amount of the scopolamine in addition to an antihistamine (though usually in a very small amount).
ピリドキシン(ビタミンB6) ‐ピリドキシン塩酸塩 pyridoxine hydrochloride
Some types contain vitamin B6, and I've actually had a couple doctors here in Japan prescribe vitamin B6 to me when I was suffering from labrynthitis (it didn't help me at all, but that doesn't mean it isn't helpful or wouldn't be to someone else).
無水カフェイン anhydrous caffeine
And quite a few of the types I looked at, though not all, contain caffeine - though this tends to be rather common in a lot of over-the-counter medicine in Japan.
To give you an example, the active ingredients in the type pictured below are 25 milligrams of Meclizine, .2 mg of Scopolamine, 10 mg of ピリドキシン塩酸塩 (Vitamin B6) and 20 mg of caffeine. (Sorry, I cut off the numbers for caffeine and scopolamine, but rest assured they are listed on the box.)
Of course, you can also try other alternative treatments, such as ginger for nausea, or even some of those wrist bands that supposedly help. (Has anyone tried the wrist bands? Do they work?) But if you're in need of actual medicine hopefully you'll now be able to find what you need.