What is the difference between "health insurance" in the US and "travel insurance"

The medical component of travel insurance is designed to deal with the immediate aftermath of an accident or medical emergency and get you home. Once you get home, the travel insurance’s job is done. Any further treatment would then be under the normal arrangements in your home country (in th UK that would mean the NHS).

Regular health insurance is designed to cover not only the immediate aftermath of an accident or medical emergency, but also the more long term impacts. There is also an element of socialism in it where, since the "affordable care act", insurers are required to provide cover for people with pre-existing conditions, even if individually those people would be a bad deal for the insurer.

@RoboKaren has a good answer but I want to expand on it a bit.

Common uses of “Travelers Insurance” in the USA consist of many factors. A lot of times having nothing to do with health care.

• Lost items – If you loose your luggage while traveling, you may get money to replace items. This can be especially important if your items are hard to come by in the US.
• Car accidents – If your in a car accident here in the US, state laws usually dictate who needs to pay what part of what. Depending on where you are when the accident happens (and of course what happened) you could end up needing to pay thousands for being hit by a car.
• Ability to return home. Lost your passport/visa. Well if you have travelers insurance you may have a much easier time while you sort out that paperwork nightmare then if you don’t. Most embassies will help you get home, but make no gaurente about speed. Specially if your funds are running low, you can’t work, and your paperwork is gonna take another 6 weeks to go through.
• Canceled Trips – This is by far the most common. A flight gets canceled and the next one doesn’t leave for 3 days (let say due to poor weather). The insurance can help with a hotel and the cost of other travel arrangements.
• Missed trips – Most travel insurance plans can help if you want to cancel or miss a trip that you scheduled, but the “vendor” doesn’t offer that as an option. I have many friends that come here to Florida and want to travel to California, until they realize just how tar away that is. Or want to visit other areas until they recognize weather, traffic, or surrounding area just isn’t what they thought it was.
• Currency issues – A bit of an odd one, but they usually provide some kind of isolation for currency exchange issues.
• Theft or other crimes – If your stuff is stolen, they can usually help get you back on your feet. In the US if your stuff is stolen your not going to be getting it back quickly. Even if the cops catch the crooks, they need to need the items in question for evidence, perhaps even for years. Insurance can help with replacement.

I will try to add more as I think of them.

Travel insurance usually covers a range of problems that might happen while traveling including lost luggage, rental car towing, and tour cancellation fees, not just health insurance.

As to whether any type of insurance meets requirements, that can vary as there are hosting agency requirements and there are government requirements:

University or hosting agency requirements vary, but for example my university requires that J visa holders (Postdocs, visiting scholars, etc) have insurance plans that meet ACA (Obamacare) requirements. Like other forms of health insurance, this would cover you you if you became sick or injured and needed longer term medical care. Note that with some plans the deductible might be quite high and might not cover regular doctors office visits, so it might not be suitable for things such as the flu or maintenance of diabetes or high blood pressure and that sort.

Many travel plans in contrast only cover emergency health care (enough to get you stable enough to travel back home) and don’t meet ACA requirements. The insurance plan itself will tell you if it qualifies under the ACA. If there’s no mention, then it doesn’t. Still check to see if they cover routine health care needs, prescription medications, and minor injuries.

As to what the US government requires for the J visa, this site notes:

Either way, the US Department of State has minimum J1 health insurance
requirements that your insurance plan must meet (which applies to J2
visa holders as well).

Medical Benefits of at least $100,000 per accident or illness
Repatriation of Remains in the amount of $25,000 Expenses associated
with the medical evacuation of the exchange visitor to his or her home
country in the amount of $50,000 A deductible not to exceed $500 per
accident or illness A policy underwritten by an insurance carrier
with: an A.M. Best rating of ‘‘A-’’ or above; a McGraw Hill
Financial/Standard & Poor’s Claims paying Ability rating of ‘‘A-’’ or
above; a Weiss Research, Inc. rating of ‘‘B+’’ or above; a Fitch
Ratings, Inc. rating of ‘‘A-’’ or above; a Moody’s Investor Services
rating of ‘‘A3’’ or above;

The same site notes that government requirements change and its best to always double check you have the latest information.

A side note: get as much health insurance as you can, even if you’re young. One of our post docs had a medical emergency and is being billed $150,000+ for a cardiovascular incident. The insurance plan this person chose was a minimal plan with a $100,000 per incident cap so they are potentially liable for $50,000+ out of pocket.

American health care costs lack any rationality.