A BattleMech, or 'Mech for short, is basically a huge, bipedal (sometimes quad), often anthropomorphic, kind-of-a-cross-of-a power armor and tank, piloted by one person. They're considered vehicles, even when they do have some features from power armors - mostly, the shape and methods of operation. They're instances of what some anime folks call mecha. The term (in this context) is specific to BattleTech game and games in same fictious universe.
The first BattleMech was seen in battlefield in February 5, 2439 (regrettably I'll probably not be making daylog at the time, so this will remain unconfirmed =) before this, there were prototypes and WorkMechs.
Typically, all BattleMechs are fusion-powered and have their own fusion reactors in the engines. The movement is enabled with myomer "muscles", which were largely idea of Dr. Gregory Atlas (who died before the first BattleMech went to war).
All BattleMechs weigh between 20 and 100 tons, having heights of 10 meters and up. For construction and damage resolution purposes, the 'Mech is divided to two parts: internal structure and armor. The old armor technology has been largely replaced by lighter and stronger ferro-fibrous armor, though these days there's reactive armor and even reflecting armor out there. All Clan designs, and these days a lot of Inner Sphere designs, also include CASE, a system that prevents damages in case of ammo explosion. All 'Mechs have heat sinks through which excess heat caused by moving and weapons is dissipated. Some 'Mechs, typically the light reconnaissance 'mechs, also have jump jets that allow them to move quickly over rough terrain and even jump on others.
BattleMechs almost always - obviously - carry weapons. Typical armament includes missiles (see BattleTech Missile Types), lasers, Particle Projection Cannons and gauss rifles (similiar to railguns), flame throwers, autocannons and machine guns. Some 'mech types, particularly those of Draconis Combine origin, have appendages for striking, known as hatchets (yeah, you guessed it, the Draconis hatchets are often sword-like in design. Katana-like, to be specific.) However, some people, particularly the Clans, consider "hand-to-hand" combat brute and prefer "less primitive" weaponry over anything else.
Other equipment includes anti-missile systems (with machine gun and laser subvarieties), radar and ECM suites, communications computers, targeting systems, artillery target aquisition systems, MASC system (that allows extra movement in critical situations with risk of leg damage), and anti-infantry pods that work like shrapnel. 'Mechs also have basic systems like navigation/datalink computers, external speaker system (and car stereo 'Mech Combat Theater®), external lights for illuminating the surroundings... and last but not least the fridge for cold drinks.
The BattleMechs are piloted by only one person, often called a MechWarrior. The BattleMech user interface includes a normal cockpit controls like joysticks, MFDs and HUDs. (As an interesting note: This stuff had not been chronicled in BT fiction too accurately - and some people laughed at Pelit reporter for making the BattleMech cockpit in one picture to look like a jet fighter cockpit. However, the imagery in MechWarrior 4 computer game seems to indicate that the cockpit really looks a lot like your typical fighter cockpit!) In addition to more conventional things, a neurohelmet provides direct brain-to-computer link. The neurohelmet is used to control the limbs of BattleMech (basically, the MechWarrior only needs to think where to move, and the metal beast moves thus) and the cockpit controls are used to control weaponry and other systems.
The equipment of a normal BattleMech is fixed. The Clans devised the OmniMechs that are a type of BattleMechs that allow different weapon/equipment configurations for same chassis.
As described above, most BattleMechs are bipedal. There are two basic types, "human-walker" and "chicken-walker" - referring to human-like and bird-like knee construction. Some random examples of human-walkers include Clint, JagerMech and Atlas, and examples of chicken-walkers include Timber Wolf, Raven and Catapult.
Tactically, each 'Mech is able to effortlessly destroy anything less "powerful" than another 'Mech of equal tonnage and weaponry. The 'Mechs are divided in four weight "classes", Light (20-35 tons), Medium (40-55 tons), Heavy (60-75 tons) and Assault (80-100 tons). Usually "normal" BattleMechs are 100% of the tonnage in specifications, but OmniMechs may sometimes use less than specified amount of equipment (some configurations may leave the thing a couple of tons short of maximum).
Whatever you do, don't insult the MechWarriors by calling these things robots. We take it personally.
(Oh yeah: My favorite 'Mech is Timber Wolf. =)
References used:
by FASA Corporation and Activision... and also some inspirational fanfics =)
Some of the Battlemechs
In case of Clan 'Mechs, I prefer to use the original Clan name. Inner Sphere designations are mentioned in parentheses.
Please /msg me the additions! Also, some of these haven't been noded, I just picked the most familiar-sounding ones from the lists =)
Light (20-35 t)
Medium (40-55 t)
Heavy (60-75 t)
Assault (80-100 t)
Other Mechlike Manifestations