Short answers, pulled from the story.
George Lucas first used the word droid in the second draft script of Star Wars, completed on the 28th of January 1975. The term is a clipped form of android which originally described robots designed to look and act like human beings.
Concept designers Joe Johnston and Ralph McQuarrie created the final designs based on influences from Dan O'Bannon and Moebius. One actor performed on stilts while wearing a costume to portray a specific character within these productions.
First class droids handle physical mathematical and medical sciences while second class droids manage engineering and technical sciences. Third class droids perform social sciences and service functions, fourth class droids execute security and military functions, and fifth class droids perform menial labor and other non-intelligence functions.
Anthony Daniels provided the voice and performance for C-3PO across multiple productions including The Clone Wars and Rebels. Kenny Baker performed R2-D2 in Episodes I through VI while Jimmy Vee took over for Episodes VII through IX.
Lucasfilm registered droid as a trademark in 1977 following the release of the original film. The term remains legally protected by Lucasfilm Ltd since that initial registration date.