The basic idea is that actions performed by player count for the entire set of characters of the same class or even entire account.
I.e. when a player gains exp, exp goes to the account-wide exp set(which adds level/skill/stat points as required to each character).
when a player (inside a skill-use to level skills system) levels up a skill, he levels up the skill account-wide.
When a player acquires currency or items, these items/currency are available account wide.