These are the series and what they mean. Please take note that Necro modules assume a party of six adventurers instead of four.
W series (dark green): this stands for "Wizards' Amulet". Thus far, there has been 1,5 modules in it: the short, free Wizard's Amulet scenario and Crucible of Freya. Both are for 1st level caharacters. They are introductory and do this job very well.
M series (purple): this stands for "Maze" after Maze of Zayene. All were written by Rob Kuntz and originally released through his small press outfit in 1987 or thereabouts. This was initially conceived as a four-parter, but a fallout between Rob and Necromancer occured, leadind to its cancellation after part 3. They are all "high level" (in 1e terms - this means 8th to 12th) and draw mixed reaction, especially from the more 3e-y crowd. I'd recommend that you check out parts one and two - these are pretty good after a few alterations.
L series (dark red):this stands for "Lairs". The adventures in the two products (Demons&Devils, Vampires&Liches) are short, extremely deadly, mostly dungeon-oriented and very close to the old Judges Guild product "The Book of Treasure Maps" in tone. I think that they are extraordinarily good and, fortunately, short.
R series (bluish green): this stands for "Rappan Athuk". Rappan Athuk is Necro's flagship product, a legendary, huge and very large dungeon. I'd recommend waiting for the compiled supermodule, which is currently undergoing editing. Levels 4th (heh heh) to 20th.
D series (drab): I think this stands for "Dungeon". Only one has been released so far, Tomb of Abysthor, which is for levels 2nd to 8th. It is my other favourite - the dungeon inside is medium-sized, but packed very densely with good stuff, including competing factions, a small wilderness section and lots of giant frogs.
DM series (light green): intended to be DM aids, the first book is The Book of Taverns, describing ten very diverse establishments in exquisite detail. All levels. The second is Raise the Dead, a must have for those who run Necro adventures... It is designed for an event when a party member dies and nobody can revive him. Four short adventures, two of which are pure genius in their feel. I'd recommend buying the book even if you aren't interested in the main shtick - they are perfectly usable in other campaigns. Mid levels (4th to 8th).
G series (olive): this stands for "Guest" (e.g. not written by Necro staff), and, not unsurprisingly, it is the most numerous series with, I think, 10 modules. They vary in their level range, feel and content. I'd recommend Aberrations (G8, lvl 6th to 8th) and The Vault of Larin Karr.
