Reptoids are a supposed alien threat that are discussed in the UFO culture as well as som conspiracy movements: they are said to work in alliance with the Greys as either equals or as the masters (in some versions they aren't allies at all) - in almost all stories involving them the Reptoids are portrayed as malevolent in regards to humanity and don't seem to shy from their plans to enslave or replace humanity.
Some people believe the Reptoids are part of an ancient race that lived on Earth prior to current humanity and seek to reclaim the world while others think they are completely foreign beings that have somehow found Earth and seek it as their own - very few believe the Reptoids to be benevolent and even amongst other supposed alien species Reptoids are often portrayed as dangerous.
If the accounts of some UFO abductees are to be believed Reptoids are not just in conflict with humanity but are also a major threat to all mammalian life in the galaxy (save for the Greys) - why Reptoids have such an intense hatred for mamalian life is not explained, though it could simply be down to arrogance or xenophobia.
According to the more radical groups of conspiracy theorists Reptoids have also managed to infiltrate humanity and are ruling the world in the form of powerful political leaders, capable of taking human shape at will and slowly working on the completion of the fabled New World Order or Illuminati.
Some radical Christain groups deny Reptoids are aliens at all but believe they are part of a Demonic Conspiracy.
Outwith the fantastical stories of New Age groups, UFO lore and conspiracy theorists the Reptoids have enjoyed more mainstream success as standard alien/monster-type in fiction: famous examples of Reptoid-inspired antagonists in fiction are The Visitors from V and the Gorn from Star Trek.