OHV = OFF HIGHWAY VEHICLE: The term OHV is used to describe a 4x4 off highway vehicle that is built by an automobile manufacture. OHV examples include 4x4 trucks and 4x4 SUV's. An OHV may also be a custom fabricated vehicle that uses many parts and components sourced from the automotive industry. OHV's may or may not be street legal. The term OHV is interchangeable with the term ORV.
ATV = ALL TERRAIN VEHICLE: The term ATV is used to describe a wide variety of small all terrain vehicles that are traditionally built by motorcycle manufactures, but not exclusively. The term ATV is used to describe 3-wheelers, 4-wheelers and side-by-sides as well as UTV's or quad's. ATV's are almost never street legal.
OHM = Off Highway Motorcycle The term OHM is used to describe a wide variety of motorcycles capable of off road travel. The most common types of OHM's are motocross bikes and trail bikes. OHM's may or may not be street legal.