A league is a broad category of abilities. In order to learn or advance an ability, a character must belong to the league that ability is from. Leagues have a monthly membership fee which increases exponentially the more leagues a character is enrolled in.
League actions are related to leagues.
The Hero League focus on melee abilities. Some examples would be slashing something with a sword, raising a shield to block an attack, or assassinating a target from stealth with a backstab.
The Marksmen League teaches ranged weapon abilities.
The Magic League is all about casting spells like fireballs to harm enemies.
The Shadow League deals with information gathering, tracking, disguise, and psionics.