In 1983, TSR rebooted the Basic D&D line with Frank Mentzer's edited edition, distinctively aimed more at new players than experienced gamers. Because of the 5 volumes in the edition; Basic, Expert, Companion, Master and Immortal, the edition is usually known as BECM or BECMI D&D, or less commonly, Mentzer D&D.
The Basic set covers 1st to 3rd level of play, breaking the rules into two books, one for players and one for Dungeon Masters. Rather than include a separate adventure module, the player's manual contains a solo, choose your own adventure type scenario, and the DM's book contains a short dungeon to introduce the DM and players to group play. Although there is no explicit mention of a default campaign world, several references in the sample adventures tie into adventures and places in the Known World/Mystara setting.
The Expert set once again details advancement to 14th level of play, as well as outdoor adventures and more information on the Known World campaign setting. Oddly, this set contains one manual, broken into player and DM sections, instead of the two manual format the rest of the edition uses. Note that some later print runs feature a much lighter blue, almost teal cover, but the artwork remains the same. This boxed set included the adventure module X1: Isle of Dread.
The Companion set continues expanding play options, opening things up to 25th level of play, as well as introducing the War Machine mass combat rules, notes on creating and administering player dominions (nations), advanced rules for unarmed combat and tournament events like jousting, and the idea of high level class variants, the Druid, Ranger, Paladin and Avenger, and more. All these options make this volume useful even to campaigns that have not reached or do not plan to reach high level play. The rules also introduce an option called attack ranks, which allows demihuman characters to improve a bit beyond their level limit.
The Master's set finishes out normal character advancement to 36th level of play, as well expanding the War Machine rules to include siege warfare. Also covered are the rules for advancing a mortal character to immortality, and rules for adventures on the outer planes.
The Immortal's set follows up on the rules in the Master's set for immortal (read: godlike) characters, and also includes further information on planar adventuring and immortal level monsters. If you're not interested in extremely high level play where the PCs are essentially gods, it's safe to skip this set.