Species name
The aliens of the UFO TV series never revealed a name for their species, nor were they given one by any humans. They were always simply referred to as “the aliens.”
Attitude toward humans
The aliens are generally hostile, or at least callous, to humans; as they come to Earth for the sole purpose of covertly capturing humans, harvesting their organs, and transplanting these human organs into their own humanoid bodies.
Not all of their species appear to have this attitude, however. There has been one incident of a "defector" alien trying to supply humans with information to help humans defend themselves.
Episode, defector alien: A Question of Priorities
Embodiment / Morphology
The exact relationship between the aliens and their humanoid bodies remains a mystery. It is speculated that each of their humanoid bodies are completely comprised of human body parts, as is in fact the case with one of the aliens who is captured and dissected by the Earth defense organization known as SHADO (Supreme Headquarters Alien Defense Organization). This leads one to wonder whether the aliens ever had humanoid bodies of their own to start with. Also, since the aliens possess a significant, yet not total, ability to control the minds of selected normal humans, one must wonder whether the aliens are merely mind-controlling the humanoid bodies that they appear to animate as their own bodies. Where are the aliens' minds? Are they in their Frankenstein humanoid bodies, or back home on their homeworld, doing remote mind control? Or, who knows where?
Whatever the case, the aliens still need human organs to maintain these humanoid bodies because perhaps their own race is somehow dying and needs these bodies to survive.
During their trip to Earth and while on Earth, these humaniod alienized bodies breathe a green colored, oxygenated liquid encapsulated in the helmets of their red space suits that they rarely remove. Apparently, breathing liquid helps their lungs tolerate the extreme g-forces of their deep space travel. When their helmets are removed, the aliens' skin retains a green tint from this fluid. Also, the aliens' eyes are covered contact-lense-like plastic shells with pupil holes to see though. Maybe this protects their eye scleras from the fluid as well.
With a few exceptions, the aliens (or at least their humaniod bodies) prefer to continue breathing the green liquid in their helmets while on Earth because they cannot long survive exposed to Earth's atmosphere.
Strategies against humans targeted for organ harvest
Covert abduction
The primary strategic goal for the aliens is to simply abduct humans to harvest their organs. Yet they generally do this covertly, likely so as not to make their presence and agenda well known to the general human populace, as such general knowledge would likely stir a panic and a stronger public resistance against the aliens.
The aliens prefer to simply make their journey to Earth in their small spacecraft, covertly land, abduct an unwitting human, place the human in a preservation casket, and return to their home planet to do organ harvesting.
Triggering mass-kill events and reaping large quantities of human bodies
Apparently, the aliens don't need freshly killed humans in perfect shape, because one of their strategies is to trigger events that would kill thousands or more humans, presumably so the aliens could take advantage of the havoc and acquire multiple bodies. Known instances of this are their attempts to trigger large earthquakes by detonating super-bombs (some of these attempts successful), and their attempt to rupture containers of a gas so poisonous that it would kill every living thing on Earth.
Episode, bomb triggered earthquake: The Long Sleep
Episode, poisonous gas: Destruction
Strategies against the secret Earth defense organization known as SHADO
Mind controlling humans to make them attack SHADO
As mentioned previously, the aliens can control the minds of a few select normal humans, apparently to varying degrees and durations. The mind controlling never seems permanent, but rather intermittent, often with the subjects not remembering having been controlled. During control, some humans seem completely under alien control. Others seem only mostly controlled, with a glimmer of their own autonomy left. But sometimes the mind control is a mere adjustment to a person's character, slowly driving them toward performing a task desired by the aliens.
The aliens often control the minds of civilian humans to make them infiltrate and damage SHADO facilities and/or kill SHADO personnel. Other times they control the minds of SHADO personnel toward these ends.
Making mind-controlled humans into bombs
The aliens can somehow manipulate human physiology to make humans into bombs that detonate when the human bomb touches a source of sufficient electric voltage or current. A few of such mind-controlled human bombs have destroyed some SHADO facilities by infiltrating them and grabbing the exposed ends of fat wires inside electricity utility panels.
Episode: Psychobombs
Threats to loved ones
The aliens manipulate select humans through threats, for example, threatening to harm their loved ones.
Episode, threats to father of navy general's secretary: Destruction
Episode, threats to wife of SHADO officer: Flight Path
Bribes
Sometimes the aliens will bribe people to subvert or attack SHADO.
Episode, bribe of SHADO officer: Timelash
All out attack via spacecraft weaponry
The alien spacecraft have destructive energy projectile guns, enabling the aliens to (as they once did) launch an all out attack via a fleet of their small spacecraft. Such attacks most likely target remote SHADO facilities, such as SHADO's military base on the moon or orbiting satellites. Again, I presume the targets must be remote so the general public won't panic and make things more difficult for the aliens.
Warping time ahead of their attack spacecraft
The aliens can speed up time within a certain distance and trajectory ahead of one of their spacecraft. It is unclear whether this ability is attached to the spacecraft itself, or is attached to something, or some person, located at the spacecraft's intended target. SHADO's headquarters in the U.K. was the target of such a strategy.
Episode: Timelash
Spacecraft
Alienized bodies travel to and from Earth in small, rotating cone-shaped spacecraft apparently large enough for no more that 3 or 4 humanoids. Although, some fan art of these craft suggest they are bigger and can hold maybe 10 or so. The destructive energy projectile guns mentioned above do not appear to be stationary, linear shaped emitters, but rather some kind of energy field surrounding the craft that can be triggered to shoot the energy projectiles in almost any direction.
It appears that at least some of their spacecraft have smaller "escape pod like" craft inside them that can deploy from the larger craft.
Episode, escape pod shown: A Question of Priorities
Weaponry
As noted above, their spacecraft have destructive energy projectile guns. The only other weapon devices they have appear to be rifles that shoot material bullets, just like conventional human rifles.