Not all abilities are available in the Learn command. Ranks are race themed and they give a combination of different abilities to a person who has a rank.
Certain higher up ranks have abilities which can not be self learned but can be taught to other players.
On RP (Roleplay) servers you normally get ranks from the admins.
On PVP they usually have auto rank on which means that if nobody online currently has that rank, and you are the appropriate race, you will get the rank.
You can not have more than 1 rank on a character.
Here are some examples of ranks:
Puran Elder: Can create lizard spheres which if all 7 are collected will grant wishes to the person who uses them.
Korin: Can create Magic Food which accelerates healing and can instantly wake up a knocked out person.
Crane Hermit and Turtle Hermit: Both possess powerful beam techniques that can not be learned anywhere else.
Daimao: A demon lord of hell. They can make special amulets which can open a portal to hell that will suck all nearby people and objects into it.
Kaioshin: A deity that is the protector of the afterlife as well as the living world should it need it. They possess the ability to revive people, let them keep most of their power while dead, restore someone's youth, bind someone to hell, and many other crazy powers like that.
North Deity: Can teach the Genki Dama (the most powerful attack in the game if you can make it hit someone) and the Kaioken ability which lets you powerup much faster than normal but instead of just your energy draining your health will drain too.