Regulation D, also known as Reg D, is a set of rules that allows the person offering the sale of securities to do so without registering them with the Securities Exchange Commission. Rule 501 and 502 contain conditions that must be met to use the exemptions under Regulation D.
The other common offering is Rule 504, Rule 506, and the seldom used Rule 505. Regulation D pertains to securities law. If a company does a private placement memorandum they usually do a Regulation D 506 or 504 public offering.
A Private Placement usually relies on a Reg D exemption.