All freeways are highways but not every highway is a freeway. A freeway is a "controlled-access" highway that's designed exclusively for high-speed vehicular traffic. Traffic flow on a freeway is unhindered because there are no traffic signals, intersections or any at-grade crossings with other roads, railways, or pedestrian paths.
The main difference between freeways and multilane highways is that in the case of freeways, these roads are separated from the rest of the traffic and can only be accessed by ramps. These ramps (slip roads) allow for speed changes between the freeway and arterial thoroughfares and collector roads. Opposing directions of traffic on a freeway are physically separated by a central reservation (median), such as a strip of grass or boulders, or by a traffic barrier. Traffic across a freeway is carried by overpasses and underpasses.
Around the world, freeways are known by various terms like Autobahn (Germany), autopista (Spain), autostrada (Italy) and snelweg (The Netherlands).
|Ingress and egress||Regulated via slip roads (ramps)||From intersections or ramps|
|Intersections or traffic signals||No||Possible|
A highway that's not a freeway may also have long stretches that are without any intersections. But highways typically have lower speed limits, may pass through inhabited areas where there are traffic signals, pedestrians or other slower traffic.
In the U.S., state highways are maintained by the respective states but interstate highways (virtually all of which are freeways) are maintained jointly by the state and federal government.
Here are some interesting pictures of freeways from around the world.