The variations in Castiel's hair through the seasons are not as noticeable as those in Sam's Hair, both because Sam's is longer and because Castiel's style is much more chaotic. It is a rough, fluffy, un-parted look. In his earliest appearances, his feathered coif was unpredictable. As Season 4 progressed, his hair settled into a pattern: longer and gelled back in front, spiked and pointing slightly forward at the sides. It was this which was dubbed his sex!hair, a term popularized in a picspam meta called The Sex!Hair of Castiel, a very scientific and intellectual study by darksilvercat.
The death of the sex!hair was mourned during Season 5, although it reappeared late in Season 6. It has been observed that the less human Castiel is, the sexier his hair.
Castiel returned in Season 5 after his first death with shorter, but no less variable, hair. It was usually swept up or back, sometimes spiked, sometimes with a slight curl. In Season 6, his hair was even shorter, and somewhat more orderly. When he ceased to be an angel in the season finale, his hair had rarely looked more precise.
Castiel's hair should not be confused with the hair of his vessel Jimmy, or that of his possible future self, both of whom have much flatter hair.