N/S/D/M/A: abbreviation of Normal/Shiny/Dark/Mystic/Ancient; more information on them found here. Order of letters may differ.
S/D/M/A: See above. Indicates only the rarer versions of Pokémon.
Unobtainable: A Pokémon that cannot be found or caught through normal gameplay (random encounters on maps). Are valuable and usually powerful.
Unob.: Abbreviation of Unobtainable (see above).