All of the standard sentences have a two letter prefix that defines the device that uses that sentence type.(For gps receivers the prefix is GP.) which is followed by a three letter sequence that defines the sentence contents.In addition NMEA permits hardware manufactures to define their own proprietary sentences for whatever purpose they see fit.
A full copy of this standard is available for purchase at their web site.
None of the information on this site comes from this standard and I do not have a copy.
Anyone attempting to design anything to this standard should obtain an official copy.
GPS receiver communication is defined within this specification.
Most computer programs that provide real time position information understand and expect data to be in NMEA format.
This data includes the complete PVT (position, velocity, time) solution computed by the GPS receiver.
The idea of NMEA is to send a line of data called a sentence that is totally self contained and independent from other sentences.
There are standard sentences for each device category and there is also the ability to define proprietary sentences for use by the individual company.
This site is based on personal research and is believed to be accurate but there is no guarantee that any of the information is correct or suitable for any purpose.
I have been told by the NMEA folks that my information is old and out of date.
The current version of NMEA at the time I wrote this is 3.01 which is not described here.