Kithkin is a creature type.
A quick and agile humanoid or elf-like species, which used to live in Amrou Haven. As of Lorwyn, not much has been detailed about the physical makeup of Kithkin. For that matter, aside from their love of telling tales, not much is known about them at all. This may be due to Wizards of the Coast trying to identify them beyond traditional roles of humans or dwarves. They have some limited magical skill, including the ability to use small creatures as messengers and enhance their weapons with elemental magic. Their most unique trait is the thoughtweft, somewhat similar to a hive mind except actual thoughts aren't shared. Feelings are shared and intensified among all local kithkin, and its power can stretch for a significant distance. This link is not invasive, however, so an individual kithkin is capable of hiding their thoughts/feelings from others should they feel so obliged.
They are aligned primarily to White, secondarily to Green in Lorwyn block and blue in Shadowmoor block. Because of this and their small nature, they have taken a role as the white weenie race of Lorwyn. With many good creatures at 1 or 2 mana and their lord creature only costing 2 mana, they have a reputation for being quick even for white weenies.
Kithkin are found primarily in Lorwyn block and Shadowmoor block, although some older cards make references to them (such as Kithkin Armor). The original Kithkin card was Amrou Kithkin, and the flavor there was that it was so small it could run between the legs of bigger creatures (power 3 or greater). Time Spiral block directly preceded white, and since Wizards of the Coast designs the future block even as the current one is under construction, they used this opportunity to preview (and essentially review) the Kithkin creature type in Time Spiral and Future Sight.