Eurodollars are the most common form of eurocurrency; they are US dollar deposits held outside the US.
There is no difference between eurodollars and other eurocurrency, except that the US dollar is the currency most commonly held outside its country of issue.
Historically, eurocurrency markets started with US dollars. During the cold war the Soviet Union did not wish to take the risk of holding dollar deposits in the US that could be seized or frozen. Soviet banks instead deposited dollars with banks in London and Paris.