Demerara rum is a type of rum made in Guyana, on the northern coast of South America. English-speaking Guyana has been a sugar producing country since the 17th century, and was the original rum supplied to the British Navy. Today, there is only one active distillery left in Guyana, owned by a company called Demerara Distillers.
Demerara rum is most similar in style to Jamaican rum, but has a smoky, almost burnt flavor that can add complexity to cocktails.
Demerara Distillers makes dozens of different rums, in a range of age statements. The interesting thing is that each individual still has a marque identifying it. For example, El Dorado 12 is a blend of ELCR (Elmore, a wooden Coffey still), SV (Diamond, a metal Coffey still), and AW (Port Mourant, a double wooden pot still). Coffey stills are column stills, which producer a cleaner, lighter style of rum.
For more information about Demerara still marks, it is best to search online -- the information is out there.