The High Priest is required to do all the services in the Temple on Yom Kippur. The High Priest was the only person who was allowed in the Holy of Holies on this day.
Every priest had to wear four garments (a tunic, a girdle, a turban, and breeches reaching from the hips to the thighs) during a Temple service. High Priests, however, had to wear four additional garments (the ephod, the meil, the hoshen, and the tzitz) when he preached in the Temple. *see below for more information*
It was the High Priest's job to keep the other priests in order. The High Priest could participate in ordinary priestly duties, but had the privilege of certain functions only meant for the High Priest, himself.
The High Priest would offer a sacrifice of atonement to God for his congregation and himself. He would enter the Holy of Holies on the tenth day of the seventh month to atone the people's sin. Jesus is compared as the High Priest in the New Testament.
From the Tribe of Levi, like Aaron (the first High Priest)
The role was usually passed from father to son, until kings started to appoint the High Priests.
Although no age was set biblically, ancient records show that the appropriate age to become High Priest was around 20 years old.
Could not have any physical "blemishes", meaning he could not be blind, lame, etc.
How were they different from other Priests?
The High Priest was above all other religious leaders. He was the most wise, dignified man who ruled above all the other priests. He was chosen by God. He could only enter the Holy of Holies to atone for the congregation's sins.
The Lord said to Moses: "Send for your brother Aaron and his sons Nadab, Abihu, Eleazar, and Ithamar. They are the ones I have chosen from Israel to serve as my priests. Make Aaron some beautiful clothes that are worthy of a high priest. Aaron is to be dedicated as my high priest, and his clothes must be made only by persons who possess skills that I have given them. Here are the items that need to be made: a breastpiece, a priestly vest, a robe, an embroidered shirt, a turban, and a sash. These sacred clothes are to be made for your brother Aaron and his sons who will be my priests. Only gold and fine linen, woven with blue, purple, and red wool, are to be used for making these clothes."