Rat bikes are motorcycle

Motorcycle

A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...

s that have fallen apart over time but been kept on the road and maintained for little or no cost by employing kludge

Kludge

A kludge is a workaround, a quick-and-dirty solution, a clumsy or inelegant, yet effective, solution to a problem, typically using parts that are cobbled together...

 fixes. Survival bikes look similar to but are different in purpose from rat bikes — they are modified for stylistic reasons.

Rat bikes

The concept of keeping a motorcycle in at least minimally operational condition without consideration for appearance has probably characterized motorcycle ownership since its earliest days. The essence of a rat bike is keeping a motorbike on the road for the maximum amount of time while spending as little as possible on it. This calls for adaptation of parts that were not designed to fit the model of bike in question. While the origin on the term ratbike is unclear, it may attributed to custom motorbike magazines, and retrospectively applied.
Most Rat bikes are painted matte black but this is not a requirement.

Survival bikes

Survival bikes are bikes that may appear to be rat bikes, but are not. They are influenced by the Mad Max films.
The term survival bike itself originated in the British motorcycle press particularly Back Street Heroes

Back Street Heroes

Back Street Heroes is a monthly UK custom bike magazine that helped to popularize a "new breed" of custom motorcycle, distinct from previous choppers because they combined rat bike-influenced utilitarian and minimalist design with greater use of high tech gadgetry, but catering to an upscale buyer...

, and the now-defunct AWoL in the late 1980s and early 1990s.