There were officially two years of Jem dolls, 1986 and 1987.  Every doll released in the first year was re-released the second year with at the least minor differences, but usually totally revamped.  A lot of dolls were released the second year only (1987, the 2nd year was the big year for Jem).  The packaging was different for each year, which is the main way to tell some of the dolls apart.  For more information on packages, see Jem Ephemera.

There were many different sets of fashions released.  Some were released only the first year, some were released both years, some only the second year.  The packaging differed- the first year fashions only had illustrations on the packages, the second year outfits had photographs of the dolls wearing them.  Generally speaking, second year fashions are worth more, especially those only released the second year.   

The Jem playsets fell into a few categories; #1 the Car, #2 the large "air" instruments for children to play that converted into a doll playset and #3 accessories and backdrops to use with the Star Stage called "Video Madness Playsets".  The car came out both years in different incarnations, and the rest of the playsets were sold both years or just the second year.