All of the actions available to your character can be found in the actions tab. If you wish to assign an action to a Quickslot, simply click and drag the icon to the free Quickslot.

Attack Selected Target
Toggle Walk/Run
Toggle Sit/Stand - while sitting your HP and MP regenerate at a faster rate
Pick up Nearest Drop/Item
Exchange Items/Gold with Another Player
Toggle PK Mode On/Off - when your PK is on you will see a crossed swords icon next to your name tag
Map and Map Menu
Merchant Mode - takes you to your shop inventory
Individual Store - currently inactive
Guardian System - takes you to the guardian or apprentice menu
Pet Summon - summons your pet or mount
Pet Return (mount only) - allows you to teleport to the nearest town
Pet Resting (mount only) - this is the same as the sit/stand action
Pet Speed Move (mount only) - this is the same as the walk/run action
Pet Pick-Up - your pet retrieves drops/items for you