Mistletoe is a parasitic plant that grows up and through and attaches itself to trees, especially hardwood trees like oaks. The roots actually penetrate the branches and suck the nutrients from the tree that hosts it, in some cases damaging the tree to the point of deformity.
There are over 1000 species of mistletoe, but only two that are native to the US. Most grow in more temperate or tropical climates.
Mistletoe is poisonous to humans, but is a food source for birds and other animals.
The Druids considered mistletoe to be sacred and powerful for its medicinal and mystical powers. It has long been considered an aphrodisiac and fertility enhancer.
And why do we kiss beneath it? The reason isn't clear. The practice may be based on ancient tradition that says that if enemies meet under a tree containing mistletoe, they must put down their weapons, embrace and call a truce until the next day.