Originally, the imperial stormtroopers started out as clones. Over time, though, the clones were replaced by recruits to replace them due to their being less susceptible to manipulation than the clones. They continued to be the Emperor's own personal army, though, remaining loyal to Palpatine until the Empire fell.
The recruits that replaced the clones continued to use the same standard armor that started with the clones. This is the most commonly seen battle armor that is uniformly white where its hard. Its actually an 18 piece armor set that covers the entire body with no distinguishing marks other than the rank of the individual, and is known as Phase III armor.