Are PRC and MOBI files identical?

( http://wiki.mobileread.com/wiki/MOBI )

MOBI is the format used by the MobiPocket Reader. It may have a .mobi extension or it may have a .prc extension. The extension can be changed by the user to either of the accepted forms.

[...]

MOBI format was originally an extension of the PalmDOC format by adding certain HTML like tags to the data. Many MOBI formatted documents still use this form. However there is also a high compression version of this file format that compresses data to a larger degree in a proprietary manner. There are some third party programs that can read the eBooks in the original MOBI format but there are only a few third party program that can read the eBooks in the new compressed form.

There are no sources quoted as far as I can tell, but the take-away is that .MOBI is an extension or prc and some features are not supported by most readers.

They do use the MOBI format, but PRC files MAY have 'Digital Rights Protection'. If they have DRM protection they won't be usable without the DRM key.