Hot rods are typically old, classic American cars with large engines modified for linear speed.
The origin of the term "hot rod" is unclear. A possible origin includes replacement of the camshaft with a new ("hotter")
version, sometimes known as a hot stick or hot rod. Roadsters were the cars of choice because they were light, 
easy to modify, and inexpensive. The term became commonplace in the 1930s or 1940s as the name of a car that had been "hopped up" by modifying the engine for higher performance. A term common in the early days was "gow job". 
This has fallen into disuse except with historians.

The first hot rods were old cars (most often Fords, typically Model Ts, 1928–31 Model As, or 1932-34 Model Bs), 
modified to reduce weight. Typical modifications were removal of convertible tops, hoods, bumpers, windshields, 
and/or fenders; channeling the body; and modifying the engine by tuning and/or replacing with a more powerful type.
Speedster was a common name for the modified car. Wheels and tires were changed for improved traction and handling. 
"Hot rod" was sometimes a term used in the 1950s as a derogatory term for any car that did not fit into the mainstream. 
Hot rodders' modifications were considered to improve the appearance as well, leading to show cars in the 1960s 
replicating these same modifications along with a distinctive paint job.