Short Answer:
Martin Guitar acoustic guitars utilize different number and letter codes to designate different body shapes, material specifications, and appointment styles.
Long Answer:
Everyone's favorite part of shopping around for Martin acoustic guitars, the naming schemes, give you quite a bit of insight about each guitar's design right on the label. With a couple pointers, you can become a much more informed player at every step.
For Martin acoustic guitars, we have a shuffle of a few letters that appear commonly. Starting out, we have "D", "J", and "M". "D", perhaps the most frequent letter you'll see in Martin's catalog designates a Martin with a dreadnought body style, the original Martin Guitar body style. "J", as in "J-40," designates a Jumbo body style. "M", goes in the inverse direction with a slimmer body depth with a 25.4" dreadnought scale length. These body style codes include lesser-seen models like "OM" for Martin's Orchestra models that comes fitted with a 25.5" scale length as opposed to the usual 24.9".
As we move into a few numbers, we get some insights into common materials and appointments. The 18 in a Martin classic like their "D-18" refers to a standby specification, a pair of mahogany back and sides along with faux tortoise body binding. A 28 on your Martin D-28 lets you know that your Martin includes two-piece rosewood back and sides and antique white binding.
Martin Guitar maintains a long legacy with many, many models and distinctions. Reading up on the different distinctions and codes will make you a more informed player!