Just think of it this way:
A straight SAE30 will not (or may not) meet the cold flow requirements for a 0W, or a 5W or a 10W. But a 0W-30 could be labeled as a SAE30 if an oil company wanted to do so. (Misleading, no?)
But both should be within the same viscosity range at 100°C.
SAE30 is a monograde oil versus 0w-30 being a multigrade. Any label that has the 'w' means it's a multi-grade, also known as multiviscosity. From the glossary off bobistheoilguy home page:
'Monograde' (single grade) is a term used to describe an oil when its viscosity falls within the limits specified for a single SAE number.(SAE Standard J300)
'Multigrade' is a term used to describe an oil for which the viscosity/ temperature characteristics are such that its low temperature and high temperature viscosities fall within the limits of two different SAE numbers. (SAE Standard J300)
the threads above reworded these definitions in plainer english. Monogrades are commonly labeled just 30, SAE30, 30HD, SAE30HD, and so on with no 'w' in the label.
In addition to all that was written above, straight-weight (aka "monograde") oils usually do not (or maybe never do) contain Viscosity Index Improvers (VII's) or Friction Modifiers.
On one hand it means they don't flow as well as their OW-, 5W-, 10W-, 15W- or 20W- brethren at low temps, but on the other they do not shear down to a lower viscosity with use, as do the xW- versions, and they are usually less volatile, if they are made with Group II basestocks, as are Delo 400 straight grade oils.