In mythology, the hero's journey begins with the "call to adventure." Destiny's herald is usually someone or something fairly ordinary--a frog, a deer in the forest, or in this case a humble droid--that carries an important message for the one who is prepared to receive it.
As the Star Wars story begins, a battle in space rages between the evil powers of darkness (the Galactic Empire) and the forces of good (the Rebel Alliance). Princess Leia sends a plea for help to Jedi Knight Obi-Wan (Ben) Kenobi on the planet of Tatooine. The hand of Fate, in the form of Jawa traders, brings her message to Luke Skywalker, a young farmboy. When Luke sees the message hologram, he is drawn into a quest to rescue the Princess and ultimately to save the galaxy.
--from the Smithsonian introduction
This is the outfit Leia wears throughout most of A New Hope.
Quite a few different R2-D2 props were built for the Trilogy. Although it is unclear exactly what function this particular Artoo peformed, it might have been used as a "costume" of sorts, inside of which Kenny Baker would have performed, or as a remote-controlled unit for shots in which the droid was to be seen rolling.
C-3PO represents the second half of the Trilogy's famous droid tandem. Looking at the gleeming bronze costume, its hard to imagine that someone (Anthony Daniels) actually fit inside of it.
Launched from Princess Leia's starship, the Rebel Blockade Runner, this escape pod barely survives Imperial detection, and safely transports the Droids to Tatooine where they being their adventures.
What is there to say? These white uniforms became one of the Trilogy's signatures, their appearance having been imitated countless times by creatively-minded fans in need of Halloween costumes.
The interrogator droid is one of the more ominous of the Trilogy's many mechanical elements, and, as we never actually get to see just what it does, one of the more mysterious. As was the case with many of these props, this particular piece was constructed from whatever materials the model-makers had on hand: one half of the droid's spherical body is actually a recycled R2-D2 dome, while included among the droid's menancing details, is a pair of forceps (visible at top).