The 0's and 1's are stored in a Cobalt and Gadolinium solution. As the name suggests once written to the information is there to stay. An even newer development are rewriteable Cds where new information can be written over the top of old information. The bits are stored in Cyanine or Ptalocyanine, which appears blue but when a gold protective film is applied it looks green. These discs are still written to by laser.
Rewriteable discs have a complex structure, see diagram.
Once the data is recorded onto disc it is read back by a laser. There are 2 ways of moving a laser across the disc the first is where the laser head is on a truck which moves along rails over the disc and the second is where the laser head is mounted on an arm which pivots across the disc. The disc is read from the middle outwards and rotates at 200 rpm at the start. A close - up of a Compact Disc being read. The red item is the laser and the pits on the disc itself are clearly visible.