EPROM - erasable programmable ROM.
  32 KiB, 64 KiB common, 4 MiB available.

  Uses a "floating gate" - a way to maintain the gate on or off without
    drawing power.
  
Source

  Chip comes with all gates set to a single value, usually 1.

  Each chip programmed using high voltage.
    Individual bits set by charging the floating gate.
 
  Chip can be erased and used again. 
    Erasure by ultraviolet which allows the charge on floating gate to drain.

  All of chip must be erased.

  Quartz window allows UV erasing, covered with paper label to protect.
   
   Source


  Both the memory cell and window are more expensive to manufacture.

  OTP (one time programming) - chips without the windows to be programmed once.
    (slightly cheaper).

  Once programmed, good for 10-20 yrs.

  Erasable a couple hundred times.
    ROM BIOS for early models or small scale systems.

  May be erased by X-ray.