The Core Rules CD-Rom was a tool for Advanced Dungeons and Dragons Second Edition produced by Evermore Entertainment, Inc. There were two editions of the rules, both of which were used for AD&D 2ed. However, the second edition included some different books, and had newer versions of some of the supplemental software. There was also an expansion produced for the second, which included the text of the "Complete Book of" rules. (Back then, the lack of these from the beginning soured my friends and I to the Core Rules CD-Rom in the first place. Almost no one played Second Edition without kits and I never found a group that played with the "Players Options" books.)
In addition to the books, each came with some software. There was a dice roller program, which was pretty, but completely unneeded. (Then again, for the amount of effort it takes to create one, it didn't hurt anything.) There was a DM-toolkit, which allowed you to generate random NPCs, treasure, and also to create your own monsters and treasure. (There are a lot of customization features, which has caused a ton of user-created add-ons to be available out on the web.) Finally, there was a map maker program produced by ProFantasy Software, makers of Campaign Cartographer 2. The mapmaker program is what you'll hear a lot of people praise most heavily when they talk about the Core Rules, however it is mostly a stripped down version of CC2. (I personally tend to use AutoRealm nowadays, or revert back to pen and paper.)
The first edition of the rules included the full text (in Windows Help format and RTF format) of the following books:
The second edition of the rules included the full text of the books in the first edition and also the following books (in Windows Help, RTF and HTML):
Later, they produced an AD&D Core Rules CD-ROM 2.0 Expansion which included the full text of the following books (in both RTF and HTML):
The Complete Book series is almost completely included in the expansion pack. They also have the Complete Book of Necromancers available for download off of their site. Just looking at it casually, I notice that the Complete Book of Humanoids and Complete Psionics Handbook are missing, which are something that my players would actually have used, had I not stopped playing Second Edition back then.
Wizards has produced a new tool which is updated for use with Dungeons and Dragons Third Edition which is created by Fluid Entertainment Inc caled Dungeons & Dragons E-Tools. This product won't include the full text of the third edition rules nor a mapmaker, but it will allow you to create characters and monsters for the third edition rules, and then print them out. (Of course, it includes a die-rolling program).