During combat, the real-time pace of gameplay is paused, and the action is split into "rounds," each of which is approximately three seconds long. During a given round, each Character has a chance to act, taking turns in an order determined by Initiative. This turn-taking helps make sense of the chaos of Combat, but flavor-wise, these actions are all happening at roughly the same time.
Most combats resolve in two to four rounds. If your World's combats regularly last more than five rounds, GMs should re-evaluate how they are setting up encounters.
In Each Round
At the start of combat, each Character rolls Initiative to determine their place in the turn order.
The result of the Initiative roll is used for the duration of Combat. Characters act in order of highest outcome to lowest outcome. Ties are broken by the Characters' rating in Dexterity, then Wits, then by the GM's discretion. A negative outcome on an Initiative roll has no result other than a poor position in the turn order.
A character who is ambushed or caught flat-footed automatically loses Initiative. This is usually dictated by the tactical situation rather than dice rolls. For example, if you round a corner to find guards waiting for you with rifles leveled, you've been ambushed.
If a Character not in Combat makes an attack while undetected, it is considered a sneak attack. Generally, this requires succeeding on a contested roll of the attacker's Dexterity + Stealth versus the victim's Perception + Alertness. A successful surprise attack grants a free attack which takes place before Initiative is rolled. A surprised target cannot make a Reaction.