A clone is a genetic replica of another being created from its DNA. Done correctly, a clone has more in common with its host than an identical twin. Done incorrectly, bad things usually happen. In spite of this shared genetic structure, a clone may develop a different personality and even slightly different features if it faces sufficiently different stimuli during its upbringing.
The planet Kamino was known for having exceptional cloning facilities, which it used to provide a clone army for the Galactic Republic just in time for the Clone Wars.
Clones form nearly the entire population of planet Conwa. They kidnap other beings, usually humanoids, to serves as the hosts for more clones, which then must be taught normal behavior in classrooms.
Clones were immortalized on planet Earth by the Rita Retrograde song "Send in the Clones".