Phase-Change is the term used to describe the material used in some re-writable disc formats such as CD-RW and DVD-RAM.
Recording is achieved by heating the alloy layer. The layer cools either to a crystalline state or to an amorphous state, depending on the temperature to which it was heated. This state change is reversible.
The two states have different optical properties. The crystalline state is reflective and the amorphous state non-reflective. This difference is detected by the optical pickup on playback.