A Borg cube is a standard starship of the Borg Collective, and is one of the most commonly encountered vessels employed by the Borg. They appear to have several different "classes" of cube with different capabilities.
Borg cubes are exactly that: cubes. From the outside, they seem to consist of many thousands of components haphazardly arranged within a cubical frame. Although many parts of a cube are redundant, there are some unique components such as central data transfer nodes that connect the ship and its crew to the greater Collective. These unique, non-metameric components represent weaknesses that have been exploited by Federation crews fighting against the Borg. Otherwise, a cube can sustain large amounts of damage without suffering severe losses in combat performance.
Cubes have no visible engines, weapons emplacements, sensors, shield projectors, or any components at all that are identifiable out of the morass of parts that form their surface.
Borg cubes present an omni-directional threat to opposing forces. They are fully optimized to engage in combat from any direction, and any facing can act as a primary weapons battery. Weapon systems include beam weapons (possibly lasers or some type of phaser equivalent), tractor beams that immobilize enemy ships and drain their shields, and shield-draining pulse weapons.
Defensively, Borg ships are quite adaptable, able to neutralize many frequency-based weapons with apparently trivial effort. The exact mechanism by which they neutralize photon torpedoes is still unclear, but it may involve shielding interplay between the weapon and the Cube.
Borg ships are still vulnerable to superior firepower, and technologically inferior forces can still inflict damage on a cube with sufficient numbers. For example, despite using weapons which the Borg had had ample opportunity to study and neutralize, a Federation fleet still managed to destroy a cube in Star Trek: First Contact. A Borg cube is capable of defeating a fleet of 40 Federation starships without sustaining any significant damage in return (TNG "Best of Both Worlds"), but a fleet of hundreds of starships is capable of inflicting heavy damage.