The Skith were thin humanoids made of a crystal that resembled and felt like ice, but wouldn't melt when exposed to heat. Most were transparent, but some were different colours, including the red leader and purple general. They were capable of shooting ice shards from their hands. (COMIC: The First)
Named Skith technology included the Mindcore, a telepathic device which stored the sum of Skith knowledge. They also had space ships and personal transports, (COMIC: The First) and were once able to built a more primitive version of a TARDIS based on the Doctor's knowledge. (COMIC: The Age of Ice)
Originally the Skith were a race of explorers and scientists, attempting to catalogue the life of the universe. However, their method was to arrive as the first alien presence on a planet, conduct experiments, take samples, then convert and consume the planet.
The Skith ship Oppressor One invaded Earth in 1915, interrupting the expedition of Ernest Shackleton. Their discovery of the Tenth Doctor and Martha Jones made them conclude that Earth was already exposed to other aliens. The Doctor stopped their plan to consume the Earth anyway, and their ship was destroyed. (COMIC: The First)
Sometime after, the Skith's home planet was obliterated by the Crimson Hand. A portion of the Mindcore on a Skith colony survived, and used the ship Oppressor Two to hunt down the Doctor for perfection of time travel to combat the Hand. They returned to Earth and invaded Australia in 2010. There they successfully captured the Doctor and Majenta Pryce, but were defeated by forces of UNIT during a dispute between ship commanders. (COMIC: The Age of Ice)