A highway is a major arterial road within a city, or linking several cities together.
Historically a highway was any major road traveling a long distance outside of cities. In the 20th century, the word generally came to be used only for important improved automobile routes.
The United States of America has a vast network of national highways linking the different states together, as does Australia.
Some highways, like the Pan-American Highway or the European routes, bridge multiple nations.
Probably the most famous highway in the United States is Route 66, while if you follow Australia's Highway 1, you can travel from capital city to capital city almost the entire way around the whole country.
Highways usually have a higher speed limit than other roads because they have more lanes and are designed for travelling at a higher speed. Highways are a grade below freeways in importance.