NMEA is short for the US based National Marine Electronics Association who define and control a number of standards, most notably NMEA0183 and NMEA2000.  The standards are combined electrical and data specifications for communication between devices such as echo sounders, windspeed sensors, gyro compasses, satellite compass, GPS, Radar and Chart Plotters.  The standard, packages the information which is broadcast from the "talker" device to multiple "listener" devices.

NMEA0183

NMEA0183 is the widest used standard since the technology to support it is commonly available at a low price.  The speed at which data is sent ranges from 4800 bits per second to 119200 bits per second.  Some classes of device use a default rate that is higher than the standard 4800.  AIS is a good example where the default rate is 38,400 bits per second.

NMEA0183 has to be hardwired in that every device (talker or listener) has its own dedicated wiring.  This can mean a lot of wiring or expensive multiplexers and demultiplexers to combine and split out circuits.

NMEA2000

NMEA2000 is the successor to NMEA0183 and allows both multiple talkers and multiple listeners (as opposed to NMEA0183 where there can only be one talker with multiple listeners.

NMEA2000 is a faster and more technically complex standard.  It has a big advantage in that the talker and listener devices can he "hung" off a central backbone that runs the length of the vessel.

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The standard defines the physical properties of the connection technology so that approved devices from differrent manufacturers can connect together.  By providing the ability to connect in NMEA0183 devices the versatility is enhanced further and ensures backwards capability to the NMEA0183 standard.  This backbone approach considerably improves the ease of installation, saves on cable costs and installation time.

It's worth mentioning that in larger commercial vessels, Ethernet is used because even though not originally designed for a marine environment it is considerably faster, there are more engineers and technicians around who understand it and it is cheaply available.  This is hardly surprising as Information Technology finds it's way into ships in a big way.