LM: Free Life Time Map Updates. Garmin GPS units with LM on its model number means once you buy the device, you will also get a free life time map updates service, which can always keep our map update-to-date.
LT, means Free Life Time Traffic Updates. Once you bought a Garmin GPS device signed with LT on its name, you will also get a free life time traffic updates service, which can always keep our traffic update-to-date, compared with LM, LT lacks of map updates.
LMT, means Life Time Map and Traffic Updates. You will have free life time map and traffic updates service when you buy Garmin GPS with LMT signed. LMT vs LM, the better one is LMT, because LMT adds free traffic updates.
W, means Wider Screen. if you have seen a Garmin GPS marked with W, it mean the device have a wide screen(size usually rang 4.2 inches from 7 inches).
Know what those symbols means can make you have a quick and better understand on GPS model name.