Trip insurance is different in different countries. In Canada you generally buy a policy that covers you healthcare costs outside of the country, with exclusions to pre-existing conditions unless you buy a policy that covers that. In the case of Canadians, the usualy maximum on the policy is $1M to $2M and the intent of the policy is to medevac you from anywhere to Canada, because once you are here, all costs for the insurance company stops (Medicare takes over.)
You can also buy seperate coverage for cancellation, trip delay and even theft. Generally you need to buy cancellation policies within 5 days of purchasing the trip, at least here in Canada.
Try insuremytrip and access america if you are in the US. In Canada, Kanetix provides a number of quotes that usually explain the policies.
The most important to know is that if you are on medication, they won't cover you unless you have a special policy unless you have been stable for at least six months (policies differ, so you have to ask). Some don't cover theft and assume your home insurance does. Others don't cover cars and assume that your car insurance does. Just ask the questions and insure that your are covering your most expensive costs. Usually, cancellation, emergency and health. In other words, cancelling the trip, having to come home for an emergency or death and finally the cost of getting medical care outside of the country.