In Episode IV, after Luke is rescued from the Sand People by Ben Kenobi, he refers to C-3PO and R2-D2 as robots rather than droids. This struck me as odd as the latter seems to be the preferred term throughout the series. Is this use of robot a one-off or perhaps a mistake?
Through the six existing scripts, the only instance I can find any of the characters referring to them as "robots" is Luke in ANH:
LUKE: If they traced the robots here, they may have learned who they sold them to. And that would lead them home!
All other dialog refers to them as "droids".
Interestingly, the scripts for the original trilogy refer to them as "robots" in the script in all non-verbal instances. The prequel trilogy, however, refers to them as "droids" in all instances.
This development can probably be attributed to the fact that the term droid is trademarked by Lucasfilm, LTD.
All droids are robots, but not all robots are droids. Robot is superset of Droid. Saying C3P0 and R2D2 robots is correct, but less precise. Its like calling a car a "vehicle" instead of just a car (thanks @ValekHalfHeart for the example).
Official (LucasFilm) definition of Droid: "A mechanical being with a self-aware consciousness, as distinguished from a computer by having a self-contained method of locomotion."