NMEA, or the National Marine Electronics Association, is a non-profit organization that promotes and standardizes the use of nautical electronics. They are best known for their NMEA-xxx standards.
These standards regulate how nautical devices communicate. They are all based on RS-232 style serial communication; however, some standards use TTL or other signal levels. The protocol supports a wide variety of devices including, but not limited to:
The NMEA protocols are very popular and used extensively for pleasure cruisers and industrial purposes alike. Most current devices support some proprietary protocol; however, most will also support NMEA, making it the obvious choice in most cases.