Martin model numbers have two parts.
The letter, O, OO, OOO, OM, D, tell the size and shape of the guitar. O is a small parlour guitar, while OOO is a large guitar, the one often seen in pictures of Big Bill Broonzy. A D (dreadnought) is the shape with not much waist, often played by bluegrass or country players. The OM is the same size as the OOO, but with a different scale length. Sometimes you will see an "S" at the end of the model number. This indicates "Special" and may mean a twelve fret neck or variation in body shape. There are newer models with different letters, I belive a very large guitar called the OOOO. All models that start with O (or OO, OOO, OM, or D) will be the same size and shape, unless they have an S at the end.
There are also numbers, the most common being 18, 21, 28, 35, 42 and 45. These indicate materials and trim and, except for special models, all 18s (or 21, 28, 35, 42 or 45s) will be made of the same materials.
A D-21 and a D-28 are the same shape, but the D-21 has a rosewood fingerboard and bridge and dark binding and no volute, while the D-28 has an ebony fingerboard and bridge, white binding and a volute.
An OO-21 and a D-21 have the exact same materials, but are different shapes and sizes.
Folks who play finger-style often prefer the smaller guitars, while folks who use flat picks ofyen like the dreadnoughts. There are becoming more exceptions to this guideline.