Sarah Marshall is a TV actress who is constantly cast as a sexy, indepedant detective/criminologist/doctor with some bizarre quirk such as blindness or animal telepathy. She refuses to do nude scenes, believing she would then be under pressure to get plastic surgery.
Peter and Sarah were going steady for 5 years. When Peter stopped taking care of himself, Sarah grew frustrated with him. This led to her cheating on Peter with Aldous for a whole year before coming clean and dumping him.
Sarah and Aldous then ran into Peter during their stay in Hawaii. After Aldous dumped Sarah for obsessing about Peter, she convinced herself that she was still in love with him and tried seducing Peter. Peter angrily shot her down for treating him like an accessory.
In a deleted scene, Peter and Sarah meet up at the airport and make their peace with each other. In a gag scene, they ran into each other's arms and started wildly kissing, promising to never split up again.
Aldous was a promiscuous rock star that Sarah slept with whilst she was still dating Peter. They travelled to Hawaii and enjoyed a romantic holiday whilst Peter sadly looked on. After discovering how self-centered Aldous really was, Sarah began to doubt their relationship. Aldous then dumped her for being too obsessed with her ex's love life.
It's revealed in Get Him to the Greek that Aldous already had a girlfriend and Sarah was just his mistress.
Rachel was the receptionist at the hotel that Sarah, Aldous and Peter were staying in. She saw how downtrodden Peter was that his ex-girlfriend had moved on, so she offered him their luxury suite for a reduced price. After that, Rachel and Peter quickly become friends. Rachel convinces Peter to come out of his shell and forget about his love life for awhile, ironically making him fall in love with her.
After talking with Sarah, Rachel sees that she's actually quite shallow. Rachel then falls in love with and starts dating Peter.