Red, Bumpy Birthmark? Does anyone know how birth marks are actually formed?

My baby girl has a bumpy birthmark on her arm. It has been identified by her pediatrician but I can't really think of how I to spell the name of it. Its bright red and bumpy. Everyone says it will disappear by the time she is five. I would just like to know what it actually is and how it was formed besides the usual "kiss from the angels."