In a nutshell, with all the codes explained.
Rolemaster started as "RoleMaster" (RM1) then became RoleMaster 2 (RM2) via a bunch of small changes.
Middle Earth Roleplaying (MERP) was created as essentially a simplified version of RM1-2.
Spacemaster (SM1) was the sci-fi variant, later followed RM1-RM2 conversion by becoming Spacemaster 2 (SM2)
In 1995 there was a major version change to Role Master Standard System (RMSS). RM2 was discontinued.
This later underwent some small changes and fixes to Role Master Fantasy Roleplay (RMFRP).
SM2 was converted to the RMSS/FRP version of the rules and called "SpaceMaster: Privateers" (SM:P).
High Adventure Roleplaying (HARP) was released, revised slightly shortly after release, but still called HARP.
RM2 was recently ressurected and slightly revised to create "RoleMaster Classic" (RMC).
HARP Sci-Fi was just released in Beta Version.
Now, current status of this mish mash of acronyms:
RM1 is no longer sold, unless you go trawl Ebey or the like.
RM2 is mostly no longer sold, you can purchase the 4 core books in PDF form at the ICE store (Click the crown in the upper left corner to go there.), but all supplements are again Ebey materials.
RMC is the most up to date incarnation of this version of Rolemaster, and is for sale here in the ICE store in PDF and hardcopy versions (You can also find it in stores and on other sites.)
MERP is dead, very dead, and only available via Ebey hunting and such.
SM1 is no longer sold, unless you go trawl Ebey or the like.
SM2 is mostly no longer sold, you can purchase the core books in PDF form at the ICE store (Click the crown in the upper left corner to go there.), but all supplements are again Ebey materials.
RMSS is as far as I know no longer in print (I could be wrong about that), though ICE and other stores/sites still have a lot of hard copy still available, and PDF versions are still available in the ICE store.
RMFRP is the most up to date incarnation of this version of Rolmaster, and many of the materials are for sale here in the ICE store in PDF and Hardcopy versions. (Some books are no longer available, so again require an Ebey hunt)
SM:P is the only version of the revised SM that I am aware of, and is for sale in PDF and hardcopy versions in the ICE store.
HARP is active and available in PDF and hardcopy versions here in the ICE store and elsewhere.
HARP Sci-Fi is in beta testing stage, and is only available here via the ICE store in PDF form.
That help? Any questions?