These are an improvement on PROM. They allow a developer to program them (sometimes referred to as "to Blow" or "to burn") with information for use with a MicroController or FPGA - and are then simply a ROM.

However, if you wish to reprogram them, then you can expose them to UV to erase their contents. The chip packages usually have a small window, which can be covered with a label to protect them from accidental erasing.