Poison oak is a plant that causes an allergic rash when touched to the skin. The substance that causes the rash is called "urushiol oil". The oil can spread if you have it on your hands and touch other parts of your body. Eventually the oil wears off (or you wash it off) and it ceases to spread.
Some people are more prone to a reaction from poison oak. The best thing is to avoid it. Corium 21 aloe healing cream can work wonders for poison oak, according to customers who have used it.