Two 17th century philosophers, Rene Descartes, and John Locke most prominently advocated this theory. The term they used was not "sense-datum" but "idea." This article does not discuss any differences in meaning that these terms might have. "Idea" as used in the theory of perception is a technical term, meaning roughly the same thing as sense-datum.
Representationalism asserts that sense-data represent external objects -- physical objects, properties, and events. But this immediately raises a question: How well do sense-data represent external objects, properties, and events? At least sometimes, they do not represent them at all well.