in the good ole us of a the interstate is a federal roadway system of freeways. in most parts of country it's  two lane in each direction seperated by a median, with limited access and a speed of 70 mph. in heavy areas it can be up to 6 lanes in each direction.

the highway is a state system mostly of two lane major roads  speed limit of about 60.

BUT it is confusing. Interstate is pretty clear but then we also have highways, freeways, expressways, toll roads and turnpikes. Where I grew up we pretty much used highway, freeway and interstate interchangably. In other words, we might call the interstate, the highway or the freeway.

Definition of a highway: A main public road, especially one connecting towns and cities.

Here is the definition for an expressway: A major divided highway designed for high-speed travel, having few or no intersections. Also called freeway, limited access highway, superhighway, thruway.

Freeways you don't have to pay on.

Toll roads you have to pay on and turnpikes (used in the Northeastern U.S.) you usually have to pay on but not always. The definition of turnpike: 1: (from 16th to 19th centuries) gates set across a road to prevent passage until a toll had been paid 2: an expressway on which tolls are collected  



Hope that helps!