BattleMechs fall into five distinct categories: Hunter-Killer 'Mechs (small, with a heavy weapon, such as a Gauss cannon or heavy energy weapon), Main Battle 'Mechs (~75 tons, with a well-balanced armour scheme and a variety of weapons covering many ranges), Support 'Mechs (large, generally mounting jump jets and long-range, powerful weapons), Recon/Strike 'Mechs (fast, light, and weakly-armed), and Assault 'Mechs. The last are the behemoths of the BT universe, slow and ponderous, heavily-armed and armoured, and able to wade into well-defended positions and come out alive. The Assault 'Mechs also usually form companies which are the basis of a battalion's hitting power, leading assaults against a specific target.
The BattleMech is powered by a fusion engine (it is unclear what fuel mix it uses), coming in standard, light, extra-light, extra-extra-light, large, large extra-light, large extra-extra-light, and compact. Additionally, the Clan-base technology has a similar range of engines, but are different. The lighter engines take up more criticals in the torso area, leaving them open to more damage than a normal engine, and allowing less ammunition and/or weaponry to be carried. Lighter engines are only good for reconnaissance and deep-strike 'Mechs, despite the apparent obsession with mounting them on heavier MBMs and assault 'Mechs. Compact engines take up less space than normal engines, but weigh slightly more.
Additionally, there are four types of gyros a 'Mech can mount: standard, compact, heavy-duty, and extra-light. XL gyros take up more space, but are lighter, while compact gyros take up less space, but are heavier. It is unclear to the author what a "heavy-duty" gyro is.
There are also five types of cockpits which can be mounted in a 'Mech: standard, command console, torso-mounted, and enhanced-imaging. Command consoles may only be mounted on >75-ton 'Mechs, and provide a link to the lance. Torso-mounted cockpits are just that--regular cockpits mounted in the chest of a 'Mech. Enhanced-imaging cockpits provide a three-dimensional view of the battlefield, allowing a MechWarrior to explore beyond his current field of vision. Finally, there are a plethora of targetting systems that may be mounted, each with its own specific job (or generalist, in the case of standard systems).
BattleMechs mount ten heat sinks (which dissipate heat generated by firing weapons) in the engine, giving a free -10 heat to the 'Mech, whatever type it is. More may be mounted, at the expense of tonnage and space. There are five types of sinks: single (-1 heat each), double (-2 heat each, with no penalty), compact (same as double, and with less space), laser, and Clan double. Laser heat sinks are a rather implausible invention, as they excite heat into visible light, and shoot it out someplace on the 'Mech.
'Mechs come in three different general types: the BattleMech, the OmniMech, and the UtilityMech. OmniMechs have the ability to swap out different weapons on a battlefield, simplifying transportation and giving certain 'Mechs a huge range of capabilites. UtilityMechs are usually unarmed, and specialized for non-combat actions, or recovery ops. There is also another distinction in BattleMechs: some stand on four legs (quadruped), while others stand on two (biped). The latter is by far the more common, despite the greater stability provided by having a greater weight distrubtion with four legs. There were briefly Land-Air 'Mechs, which were introduced by WizKids as a tack-on of some Japanese animes where transforming mecha were common. These were short-lived.
BattleMechs are piloted by special people called MechWarriors. These highly-trained soldiers are anagalous to medeival knights, in that they often find themselves fighting alone, and enjoy a higher level of comfort when piloting a BattleMech--such as a couple litres of water, rations, and other minor things that an infantryman will often lack. The MechWarrior sits on a chair in a small cockpit, referred to as the command couch. A neurohelmet connects his brain with the 'Mech's systems, providing locomotion, balance, reactionary movement, etcetera. It is assumed this is facilitated through the use of electrodes attached to the temples and other areas of the head (some depictions of MechWarriors have hair; it is unclear why this would be so).
Movement for BattleMechs is facilitated through the use of mechanical muscles, called myomers. These function exactly like organic muscles, relying on electrical impulses to contract and expand. Technicians are known for their ability to perform operations resembling surgery on BattleMechs, in which they will extract functioning myomers from a BattleMech or stockpile, and install it in the place of a torn or otherwise malfunctioning myomer.