Despite the amazing technological improvements in aircraft and air-traffic control, winter can still snarl air traffic and result in cancellations, delays, missed connections and assorted other ills. While one cancellation can appear pretty much the same as another, travelers find quite a bit of difference in what airlines owe them in a major snarl. Here's a primer on cancellations and what you can do about them. With one exception, what you get when your flight doesn't operate as advertised is entirely up to the airlines. That one exception is when an airline "bumps" you due to overbooking: when it sells...