A font designed by Hermann Zapf in 1958, based on lettering on the floor of church of Santa Croce in Florence.
Optima is a sans serif typeface - to be more specific, a humanist sans serif. It is, as the name suggests, a rather optimal in look: The letter features are almost the same width, in spirit of modern sans serif typefaces, but they get subtly narrower or thicker where it is needed - and it even has very little "beginnings" of serifs, if you look really close. It sort of tries to subtly say that it isn't actually coldly geometric, but rather designed by a human.
Computer versions of the font are available from Linotype.
Mostly based on what I heard on lectures here, but the details were either verified or learned from http://www.creativepro.com/printerfriendly/story/17185.html