Seizures are a result of aberrant electrical activity in the brain, and often it arises from a seizure focus. If it's a very small focus, it can stay localized, and you can actually have a seizure—this is also something I didn't know—in only one part of your body. If it's just a small focus in your brain and it doesn't spread, your arm could shake and you might not even be aware.
My supervisors told me stories about a guy who had his arm shake for weeks, and he didn't know why. He was actually having a seizure--for weeks.
It can stay localized, and if it does, that's when surgery is an option. If you can locate that focus, you can remove it and prevent the aberrant electrical activity from starting. However, it's more often the case that it will spread, or the focus isn't localized and is everywhere, it will involve your whole brain, in which case it would be a generalized seizure.