A back door listing, sometimes referred to as a reverse takeover, reverse merger, or reverse initial public offering (IPO), occurs when a privately held company that may not qualify for the public offering process purchases a publicly traded company.
By undertaking a back door listing, the private company avoids the public offering process and gains automatic inclusion on a stock exchange. Following the acquisition, the buyer may merge both companies' operations or, alternatively, create a shell corporation that allows the two companies to continue operations independent of each other.
Although rare, a private company sometimes will engage in a back door listing simply to avoid the time and expense of engaging in an IPO. For example, Archipelago Holdings acquired the New York Stock Exchange (NYSE) via a back door listing in 2006. A back door listing usually indicates significant weakness in the acquired company and serves as a warning sign for investors to be wary.
For more on this topic, read "Mergers and Acquisitions: Definition."
This question was answered by Justin Bynum.