You got it right. KEI is a contraction that means "light automobile ". They are a limited class of vehicle, 660cc engine, small size and no more than 64 horsepower.
They make small trucks and vans also. They are made and used in Japan and they have no problem keeping up with Tokyo traffic (which if you are envisioning bumper to bumper traffic is not that impressive). In Japan they get a huge tax break and make up 1/3 of cars in Japan.
Compared to a SxS they will last much longer but typically you are getting a used vehicle in the US. They where made with all the street legal stuff you would expect from a car (safety standards are diffrent in Japan). Off road they are not as capable stock and difficult to modify. But they do have 4x4 variants with features like locking diffs. Most of the trucks have nifty drop down sides, think 3 tail gates.
My experience with them is mostly in Japan. In the early '70s my family visited Japan. My uncle had recently been promoted to the level where he had a company car so he came to the airport to pick us up. In my memory it was a Honda N600 (they used to sell them here) but uncle was a Toyota man so that is probably not correct, but that is the size car we are talking about. Uncle put all the suitcases on the roof and lashed them on with rope about doubling the size of the car. Then us 4 Americans and Uncle piled in and off we went. Probably not the experiance you where looking for but oh well.
