See: Red Herring. It's the same thing, more or less.
An offering circule is just the marketing materials with all of the information that the bank, the underwriters, or the company deems is important for potential investors to know. Legal waivers. Financial estimates. Company history. Margins. Growth. International...stuff. It's usually all in there.
And it's "circular" in that it circulates through its investment community; those who distributed it then circle back to the readers to get a response. Usually a negative one.