Practice of shaving or cutting most of the hair on your scalp is called tonsure. It is a sign of humility and religious devotion. Tonsure in the past was only used for the description of olden days Catholicism, where the peculiar hair styles come from. In general, it is the symbol of renunciation from worldly esteem and fashion. Apparently by tradition, the disciples of Jesus Christ observed the Torah command, where you are not allowed to shave one side of the hair.
There are three types of the style commonly used back on its conception, the Oriental, the Celtic and the Roman. The Oriental consist of the now classic full bald. The Celtic doesn't cut them fully to form sort of an arc from ear to ear on the top of their head. Then the Roman style that is most often depicted in western medieval movies, where the only the top part of the hair is shaved to form a crown. So why the weird choice of hairstyles for the Celtic and the Roman… It is to resemble the head of chief apostle Peter, who on his journey to spread the teaching of Jesus, would have people who do not believe his preach to shave his head in mockery. Jesus Christ blesses his head and so changing the view of the hair to be that of honor and praise. Or so the legend goes. But yeah, shaving the head is mostly a sign of abandoning the worldly desires.