There are two broad ways for combat to begin: Mutual and Surprise.
Mutual combat is the most common kind, where two sides become aware of each other simultaneously and decide to have it out. If this happens, Initiative is determined immediately and characters and NPCs act based on that order.
Surprise combat is when one side begins a fight without the other knowing about it. This could be an ambush, betrayal, or excellent stealth. Those who initiated the combat have a free round of action called a Surprise round and can act in any order they choose. After the Surprise round of combat, Initiative is rolled and combat continues as normal. The surprised party may take Reactions during this round, but nothing else.
Round: A round of combat is a complete circuit where all characters and NPCs who are able to act take their turns.
Turn: A turn is the container representing approximately six seconds of real time wherein a character or NPC moves and takes actions if they are able. Many timers, such as a Death Roll, are incremented during a character's turn.
Initiative: Initiative refers to the order in which the characters and NPCs act. There are many ways to determine this and it is recommended that a GM discuss this with their group. This Campaign Setting prefers the following method, though this is by no means a rule:
Each character rolls a d20 and adds their Air Modifier. All normal enemies are rolled as one and a common Air Modifier is applied. Unique or powerful enemies are rolled separately. The characters and NPCs then proceed in descending order. It is also possible to roll only characters and powerful NPCs and then have all normal enemies act afterwards.