The coverage usually provides coverage for medical costs incurred during travel and in some cases also covers medical evacuation.

Travel health insurance usually comes bundled up with trip interruption/cancellation insurance, travel accident insurance, baggage loss, emergency evacuation and medical expenses coverage.  The usual premium is about 200$ annually.  However the premium depends a lot on your age, cost of trip (for single trip insurance coverage), the distance of trip, the nation to which the trip is being made, any preexisting health conditions etc.  Some travel health insurance plans also cover accidental deaths and flight accidental death.  

The plans that cover the medical costs on your trip include medical expenses, dental expenses, cost of treatment and cost of medicines. These covers come attached with lot of conditions like if you take medicines from unconventional medical sources like acupuncturists or ayurvedic treatment, the costs will not be reimbursed.

However if you are already covered by a health insurance, it is prudent to check whether it covers outside the United States or whether it provides only for emergencies.

Medical Evacuation: travel health insurance provides for medical evacuation to nearest qualified medical health facility. Here the catch is nearest medical health facility and not to the US. So if you are traveling in an underdeveloped country, you may be forced to take medical treatment at a facility decided by the insurance company and is not decided by you. The treatment in the US is one of the best and other countries including Europe have a higher surgical mortality rate.

Repatriation of mortal remains.  In case of death the travel health insurance company will bear the cost of returning the mortal remains of the insured.

There are other conditions like co-insurance, deductibles, exclusion, benefit period, pre-existing conditions etc which have to be carefully studied before buying a travel health insurance policy.