A highway will have streets entering into it at a 90 degree angle. So the cars pull out into traffic and need to get up to speed. A freeway cannot have any access points except for on ramps and off ramps where vehicles can reach freeway speeds before merging with traffic. Freeways are safer for this reason.

While there are some differences, anyone should understand what you are saying if you use them interchangeably :D

A highway might also have a toll that applies to it. Freeways, as a rule, are free to use. It also depends on the region whether one or the other is used more. In California most people say they went on the Freeway.

Only in USA Gary

are you interested in the answer for any country, or only the USA?