"Rolemaster (1st & 2nd Editions) includes the first set of rules originally published by Iron Crown Enterprises for their fantasy role-playing game around 1980 and essentially includes all the Rolemaster material that was published by the company until Rolemaster Standard System (3rd Edition) was released in 1995.
The changes between the first and second editions are minimal with a few pages of additional or optional rules added to each of the rulebooks when a few of the books were combined or formatted differently with new artwork. Further additional rules were added to this system with the Rolemaster Companion series of rulebooks which were published from the mid-1980s to the mid-1990s.
Rolemaster Classic is a reworking of the Second Edition. Most of the changes were organizational, but the tactical system was replaced with a simpler version of the RMSS/RMFRP activity percentage system."
Source: Wikipedia, "Rolemaster", available under the CC-BY-SA License.
From publisher blurb:
Rolemaster Classic is essentially a reworking of the Rolemaster 2 rules from the early 1980′s.
However, Rolemaster Classic is not exactly the same as that earlier version. There were a number of changes, both major and minor, in the reworking. All of these changes were meant to make the books easier to use than the 2nd Edition versions.
The vast majority of the changes were merely cosmetic and concerned a re-organization of the material to make it easier to read and understand. In addition to the organizational and readability changes, we also made a few other changes as well. Things like replacing the original phased tactical system in Arms Law with an activity percentage based system.