One is high, the other is free. Highway = any main road with two or more lanes. The road can have junctions, roundabouts, or, God forbid. traffic lights. Freeway/Motorway = just that. No stopping on those.

"Highway" is an old word that means any main road. The meaning of "main" lies in the eye of the beholder. A freeway is a limited-access divided road with at least two lanes in each direction and having no intersections. The "free" may not apply to cost, since some of these roads require payment of tolls.

I agree with Randy's posted answer. Here in Southern California we tend to use "freeway," unless it's part of the name like "Pacific Coast Highway." The maximum speed limit is generally 65 mph (miles per hour), though for a few stretches it's 70 mph.

In the UK it's different again. We use 'motorway' to refer to the largest roads we have in terms of lanes - usually 3 or 4, and distance you can travel on them - up to several hundred miles. These roads have a speed limit of 70 mph. These roads have no traffic lights, stop signs, roundabouts and so on. We then have dual-carriage-ways, which usually have 2 or 3 lanes and have a speed limit of up to 50 mph, however these roads have traffic lights and the occasional roundabout. Then we have normal roads, with a speed limit of 30 mph, slightly less near schools, with traffic lights, zebra-crossings, pelican-crossings, roundabout, sleeping policeman and the like!

Basically, they are often used in the same way here in the US, along with the word "Expressway". However, I believe there is actually a difference that most people ignore. In Michigan: Highways are typically a 55mph road, which usually does not have traffic lights or at least does not have them very often. Expressway/Freeway is 70mph and DEFINITELY does not have traffic lights. Even though there is a difference in their meaning, I typically use all 3 words to mean Express way. "I'm going to take the highway" = "I'm going to take the freeway" = "I'm going to take the expressway"... they all would have the same meaning in my opinion.