BattleTech is a science fiction tactical wargame in which players maneuver giant fighting robots ("BattleMechs," or "'Mechs") against each other (or vehicles and/or infantry). The game system includes details such as varying weapon ranges, heat generation, and discrete hit locations (torso, arms, legs, etc.). 'Mechs vary greatly by mobility and firepower, and players can choose from provided designs or take the time to create their own 'Mechs from scratch. The base game (reprinted in many different editions) includes an introductory rulebook for new players; recent editions also include quick-start rules for an even faster orientation. The game is typically played on one or more mapsheets or (more recently) mounted map boards, and there are many mapsheet and "hex" (mounted-board) packs available for purchase or even download.
The main component of the current generation of BattleTech is a series of core rulebooks, the central one being Total Warfare (in print and PDF), which contains the main (tournament-legal) rules for the game. (The 2011 Catalyst reprint is up-to-date in terms of errata fixes.) TechManual (in print and PDF) contains construction rules for 'Mechs and other units. A series of three "advanced rules" books add on to the standard rules: Tactical Operations (in print and PDF) covers on-planet actions over a period of hours; Strategic Operations (in print and PDF) covers in-system actions over a period of weeks; and Interstellar Operations (forthcoming; 2012?) covers system-level actions over a period of months. The final core rulebook available is A Time of War: The BattleTech RPG (in print and PDF), a revised version of the BattleTech RPG rules, which have previously gone under the names MechWarrior and Classic BattleTech RPG. A one-volume tome on the BattleTech universe was announced years ago but has no scheduled release.
BT is also supplemented by a variety of Technical Readouts, which provide gameplay and background data on 'Mechs and other units at various time periods in the game-universe history; map sets, including the new Hex Pack hard-mounted maps with tile overlays; sourcebooks; and pre-generated record sheets, as well as the aforementioned miniatures (which are not required for play).