I have read it is a font with additional characters and ligature.
The expert set was introduced it in the beginning of the 90s, see http://www.typophile.com/node/40309.
Exp | Expert Set
Depending on the foundry Expert Sets could hold different configurations of glyphs. The naming implied that those fonts provided all the characters missing in the standard fonts that a typographic expert may have need of. Originally Expert Sets included only small caps, oldstyle or hanging figures, additional ligatures, often super- and subscript letters and numbers, plus some additional special characters and sometimes swash variants. Normal height capitals were absent, and their slots were occupied by other expert characters.
I am wondering if there is specific elements that characterize an expert set beyond the fact that there is additional characters provided.