Mary is called “The Magdalene” because she was from Magdala, a thriving fishing town on the coast of Galilee. While there were other towns with similar names, Magdala is within walking distance of Capernaum, where Jesus was based at the start of his ministry. Recent archaeological findings in the current day Israeli town of Migdal, including a 1st century synagogue, support this explanation.
There are half-dozen Marys mentioned in the Bible. but the most distinguishing characteristic about this particular Mary is that Jesus cast seven demons out of her (Luke 8:2-3).