A single company may issue several classes of preferred stock. For example, a company may undergo several rounds of financing, with each round receiving separate rights and having a separate class of preferred stock; such a company might have "Series A Preferred", "Series B Preferred", "Series C Preferred" and common stock.
In the United States there are two types of preferred stocks: straight preferreds and convertible preferreds. Straight preferreds are issued in perpetuity (although some are subject to call by the issuer under certain conditions) and pay the stipulated rate of interest to the holder. Convertible preferreds--in addition to the foregoing features of a straight preferred--contain a provision by which the holder may convert the preferred into the common stock of the company (or, sometimes, into the common stock of an affiliated company) under certain conditions, among which may be the specification of a future date when conversion may begin, a certain number of common shares per preferred share, or a certain price per share for the common.
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