This is not a comboing guide - there are plenty out there. This is for newbies who want to know what it is, and the theory of how it's achieved.

Combo is used to use skills faster that your character does normally. When you use a skill your character does an animation to go with the skill, and depending on the skill used, the "action" happens at some point through that animation. Comboing is when you cut the animation short as soon as the "action" has happened, letting your character use the skill again (or a different skill if it is still on cooldown).

The most commonly used combo is for attacking, as the "action" (i.e. the damage) for most attacks happens very early in the animation. Attacking combos also make use of the basic attack "r" during the combo.

Various actions in the game will cancel your skill animation. Most often the action used is movement with the keyboard (w,a,s,d). Other possible actions include "Stop attack" (pressing r to stop a basic attack once it's been started), sitting down ©, towning (/town) or movement with the mouse (much trickier).

Many skills in the game can be combo'd, such as a priest's heals and buffs, a mage's AOE attacks, or "blade" attack. Even a mage's TP skill can be combo'd to some extent. Some skills can be combo'd more easily if the mage/priest is in a transform scroll.

The best way to learn to combo is to watch the animation of your skills carefully, and work out at what point the "action" happens. Then you have to practice moving just as soon as you reach that point in the animation. For practice purposes i'd recommend using "s" to take a step backwards, as this moves your character the smallest distance.

Historically there was a very big argument about whether combo was legal or bug abuse. Finally the GMs (in USKO at least) decided that combo was legal.

Most players now agree that combo adds a level of skill to the game where it would otherwise be all level/item based.