Captcha is the in-game anti-bot system. It has a small chance to show up when a player completes an activity, like successfully mining an ore, killing a monster, buying items from NPCs, etc. It doesn't show up when the player is idle (not moving and not clicking).
Missing a captcha or closing the captcha window will remove a captcha point from the player's "penalty" record. If the captcha points reach -5, the player will be teleported to Jail and then only a Chat Moderator or an Admin can decide to free the player. When the captcha points are from +1 to +5, these points can be assigned to a skill of the player's choice for extra Experience. The maximum captcha points that can be kept for later assignments is 5. After that, additional captcha points will be lost so it is recommended to use the fifth point on a high exp skill and use all during a 2x experience event on whatever skill you want.
The amount of experience rewarded is given by the chart below. For the same level, the reward to carpentry is 50% of the normal reward, and to breeding is 25%. During 2x experience events, the rewards are also doubled.
*For all skills except carpentry and breeding.