The Basic D&D sets comes in three revisions: The Holmes (original), the Moldvay/Cook Basic/Expert (B/X) and the Mentzer Basic/Expert/Companion/Masters/Immortals (BECMI).
What are the differences between the three? Are they as radically different as AD&D editions are?
It seems that most people who refer to Basic refer to B/X (Moldvay/Cook), and not Holmes or Mentzer/BECMI. I'm curious as to why.
they are two of 3 flavors of the Basic D&D Line. (Holmes wasn't intended to be a separate line, it was supposed to be the starter set for AD&D.)
BX is basic by Tom Moldvay in 1980, and Expert by David Cook in 1981. It's the first point where DemiHumans are classes. Basic contains Fighter, Mage, Cleric, Thief, Elf, Dwarf, and Halfling. Many printings arrived three-hole punched. It's also the edition shown in Stranger Things.... but there is no Demogorgon in it.... Basic is levels 1-3, but has table support for up to 5th. Expert is levels 4-14. It's not AD&D compatible, but is readily convertible on the fly.
BECMI: Frank Mentzer was tasked to revise and expand the D&D line in 1983.
Basic and Expert are both the same basic content. Same levels. Basic has a solo module section to kick things off, too.
Companion runs levels 15-25. It adds a number of new high level character options that backport to level 9+: Druid, Paladin, Knight, Avenger. It adds the 20-die limit on spells, too. High-experience demihumans can get ahold of Clan Relics. and also continue to improve in combat capability, but not in HD, HP, nor spells cast. A landholding system is included, as is a mass combat mechanism. It also introduces the interplanar adventures.
Master is levels 26-36. It adds a new class (Mystic), and the quest for immortality. Weapon Mastery rules are also included.
Immortal is Levesl i1 to i36
Most of the changes are minor errata issues. It's fully compatible with BX, but (except for Expert) is split into two books per box, and the books are no longer available outside the box in regular channels.
D&D Cyclopedia by Aaron Alston is the next errata version, but it covers all non-immortal levels and options in one single hardcover. Alston makes a couple subtle changes, but they can cause a few problems with rules-lawyers. Literally all the abilities from BECM boxes of BECMI are included. Overview maps of Mystara and the Hollow World are included. General Skills are pulled in from Hollow World and the Gaz line.
Wrath of the Immortals was a big-box boxed set, covering levels i1 to i36 as well. It's mechanically different than Mentzer's edition. Alston chose to fix problems at the root with this one.
D&D Basic Set (big black box) is done by Troy Denning, and covers levels 1-5. This is the flavor of basic with the cardboard standups and a full color dungeon map. There are two different covers for the included book by which printing.