As soon as anyone sees a clone, they think: "He's Bad."  It's certainly the impression I get from them.  They can morph their face to resemble anyone, which makes me wonder why they would choose to look like the mean guy. (See above picture.)  When wounded, they bleed green blood, but they can only be killed by a steel kind of knife in the back of the neck.  There are many kinds of clones.  Some are bounty-hunters, others healers and some kidnappers, it depends on the individual.

It's hard to say if they're from another planet, but I think they were originally, and are now 'breeding' on Earth.  The government knows they're there and seems to be involved in what they do, or in some cases against what they do.

A clone has healed Mulder's mother and led Mulder to his cloned Sister.

So, they seem to always be around, involved in most cover-ups and X-Files events.  They seem to get them-selves involved in everything.  Pretty nosy if you ask me!