Alderaan is an Earth like planet that was home to Princess Leia Organa until Grand Moff Tarkin ordered its destruction to persuade Princess Leia to tell him where the Rebel base was. At a stroke, over a billion lives were lost as the planet was blown up which Obi-Wan Kenobi sensed whilst he was travelling to the planet with the droids C-3PO and R2-D2. Apart from a brief scene in star Wars III - Revenge of the Sith, the planet is only mentioned and never seen. Alderaan is probably much like Naboo, the home of Padme Amidala, Princess Leia Organa and Luke Skywalkers mum.
When Anakin Skywalker turned into Darth Vader, the children of Padme Amidala were separated so that the Empire wouldn't find them. Leia was adopted by Senator Bail Organa as his wife craved for a daughter. Luke was given to Anakin's step family on Tatooine.
There's always a possibility that Alderaan could appear in a prequel movie as the planet was destroyed in Episode IV.