What is the difference between straight bourbon, blended bourbon and sourmash bourbon?

Today, all straight bourbons are produced by the sour-mash method. In the sour-mash method, backset (liquid from a previous distillation) is added to the mash in addition to yeast. The backset helps to make the next batch of bourbon similar to the previous ones by passing on some of the characteristics of the previous batches (this is similar to using a "starter" when making sourdough bread). Straight bourbons are produced in accordance to the laws mentioned in FAQ #1 What is bourbon? and contain only undiluted grain distillates. Blended bourbons contain at least 51 percent straight bourbon mixed with neutral grain spirits.