The first and most common is Trip Cancellation/Interruption coverage.
Most of these policies cover cancellations due to weather, sudden illness, death and emergencies whilst at home or abroad.

Another common type of travel insurance is for medical emergencies.
This type of coverage is particularly useful if you plan on traveling to underdeveloped countries, or for individuals with an on going illness that might require medical attention at some point during the trip. These policies will reimburse you for the cost of doctor visits, medication, and sometimes even medical evacuation out of the country. It's best to check with your insurance provider regarding the details, as travel health coverage policies vary widely.

It is also possible to obtain travel insurance that will cover non-refundable tickets in the event that a trip has to be canceled for any number of reasons.
Some coverage plans will sometimes cover missed connections as well, which can be a lifesaver in the event of a flight delay or over-crowded and/or confusing airport terminal.
For longer trips, or for frequent travelers, a comprehensive insurance package is probably the best value. These usually provide a wide variety of coverage, and some even allow you to choose what kinds of coverage to be included. Since it's impossible to know what problems might arise during your trip, these policies cover all the bases so that you have protection against monetary loss in the event of nearly any emergency.