A rat rod is a style of hot rod or custom car that, in most cases, imitates (or exaggerates) the early hot rods of the 40s, 50s, and 60s. It is not to be confused with the somewhat closely related "traditional" hot rod, which is an accurate re-creation or period-correct restoration of a hot rod from the same era.
Most rat rods appear "unfinished" (whether they actually are or are not), with just the bare essentials to be driven.
The rat rod is the visualization of the idea of function over form. Rat rods are meant to be driven, not shown off. Sometimes the customization will include using spare parts, or parts from another car altogether.
From
http://en.wikipedia.org/wiki/Rat_rod
Also this part is fitting for us Bronco dudes...
Paint and Finish
Many Rat Rods appear unfinished with primer paint jobs being common. Other finishes may include “natural patina” (the original paint with rust and blemishes intact), a patchwork of original paint and primer, or bare metal with no finish at all in rusty or oiled varieties. Many rat rods also have free hand pinstriping done by the owners with a pinstriping brush. Contrary to tastes of many car builders, rust is often acceptable and appreciated by a Rat Rodder.
Interior
Interiors of rat rods vary from fully finished to a spartan, bare bones form. Mexican blankets and bomber seats form the basis of many rat rod interiors. Most are designed to be functional without many comforts although this will vary with the owner’s taste.