Meks are large walking robotic devices that can be piloted directly by a human pilot, remotely by a human pilot, or by an AI. Meka largely fall in to 2 categories, Industrial and Military.

For current Military Capabilities and Classes see here:

History of meka
Meka were developed primarily for working in hostile conditions on other planets.  The first meka was seen in battlefield in February 8th, 2169 during the Martian rebellion, when the rebels took industrial robots, and used the industrial lasers and hydraulic equipment as weapons against the lightly armored UN troops. The results were bloody and decisive.

However, further development of military meka languished until the development of portable fusion cores in the 24C, which enabled the development of Meka, very similar in design to the Meka we see today.
General Description
 Meka weigh between 20 and 100 tons, having heights of 6 meters and up. For construction and damage resolution purposes, the Mek is divided to two parts: internal structure and armor. The old armor technology has been largely replaced by lighter and stronger nanoarmor, which can offer reactive and regenerative properties. All Meks have heat sinks through which excess heat caused by their power core and weapons is dissipated. Some Meks, typically the lighter Meks, also have jump jets that allow them to move quickly over rough terrain and even jump on others. 

Nowadays, all Meka are fusion-powered with a portable fusion core and movement is enabled with "nanomuscles", which are an inorganic equivalent of of an organisms muscle. 

Typical armament includes missiles, plasma canons, lasers, particle weapons, gauss rifles, flame throwers, autocannons and machine guns. Bioweapons, chemical weapons and nanoweapons are banned by the Terran convention. Meks do include detectors and counter measures to these weapons.

Other equipment includes anti-missile systems (with ballistic and laser subvarieties), radar and ECM suites, communications computers, targeting systems, and anti-infantry pods that work like shrapnel. Meks also have basic systems like navigation/datalink computers, external speaker system, external lights for illuminating the surroundings.and last but not least, a fridge for the beer.
Most Meks are piloted by only one person, called a Mek pilot. The Mek user interface includes a normal cockpit controls and HUDs. In addition to more conventional things, a neurolink provides direct neural-system link. The neurolink is used as the primary control of the Mek. Tactically, each Mek is able to effortlessly destroy anything less "powerful" than another Mek of similar tonnage and weaponry in battle. Against tanks Meka have a big advantage in speed & agility.
Types of Meka
As described above, most Meks are bipedal. There are two basic types, "human-walker" and "chicken-walker" - referring to human-like or  bird-like knee construction respectively. Some very heavy units, such as the Heavy Engineer and the Supply units are often quadraped  

The Meks are divided in to classes depending on weight, weaonary and special abilities.

Common Mek Models
Light Recon class: Light, stealthy jump jet capable eg Eagle recon
Light Engineer Class: Light, carry mines, repairs
?Air Defence Class: jumpjets and missiles
Snipers Class: medium with good long range weapon and jump jets eg
Command Class: medium to heavy, stealthy capable eg Coyote. br />
Attack Class: medium to heavy, good speed eg Beholder
Heavy Engineer: super heavy quadruped, engineering base
Artillery Class: heavy, with missiles and long range canon

Related: Meks and vehicles