LMT simply means Lifetime Maps and Traffic and is part of the model name for Nuvi's that come with those 2 features bundled with the GPS. There are also models that have LM only (lifetime map updates are bundled but no traffic receiver is included) or LT or T models which come bundled with a traffic receiver but not Lifetime Map updates.
By LMT I expect you are referring to Garmin's Lifetime Map Updater program which automates, to some extent, the downloading of map updates to your Nuvi. That's all it does.
Webupdater, on the other hand, is a software program that allows you to update the firmware, software, voices, and all other items on your Nuvi, EXCEPT MAP UPDATES.
Rick James - Nuvi Forum Moderator Nuvi 350 - Nuvi 760 - Nuvi 1695LM - Nuvi 3790LMT (with ecoRoutes HD) - Nuvi 2460LMT - Nuvi 3597LMTHD (with ecoRoutes HD) also TomTom 540S for side by side comparison >> Here << 2010 VW Golf SportWagon 2.0 TDI Diesel