Leia Organa started out as Princess Leia Organa in Star Wars IV - A new hope, an ambassador of the planet Alderaan.
As Darth Vader imprisoned her without a trial, the imperial way, and she witnessed the destruction of her home planet from The Death Star. At this point she was no longer a princess.
Later in this same movie, she was rescued by Luke Skywalker, Han Solo, a wookie named Chewbacca, and two droids.
After the destruction of the first Death Star in the battle at Yavin, she joined up with the alliance/rebels as a soldier, rather than a spy.
Her service record includes the battle at Hoth, where she was forced to run away with Han Solo on the Millenium Falcon, and stayed with him for some time, only to witness the abduction of Solo by Bobba Fett, a bounty hunter who wanted to collect the price on Solo's head.
At this point, she had fallen in love with Solo, and seeing him frozen in carbonite was bit of a shock for her.
She and Lando Calrissian spent quite some time locating Solo, finding him in the palace of Jabba the Hutt.
Leia was able to thaw Solo out of the carbonite, but was captured and was held as a slave by Jabba the Hutt, wearing a gold bikini she filled just the right way.
After strangling Jabba the Hutt, she left Tatooine and headed out with her party, just to discover that another Death Star had been built.
She voulunteered for the mission to the forest moon of Endor, which resulted in the attack of the shield generator protecting the Death Star. As the shield went down, the rebels were able to destroy yet another Death Star.
After the battle at Endor, she married Han Solo. She continued working for the New Republic, a new governing form that arose among the rebels, and became a famous diplomat. She also represents the Alderaan refugees in the council of the new republic.
She has three children togheter with Han Solo.