Turn vs. Round
A round is 6 seconds of in-world time — every combatant acts once before a new round begins. Your turn is your personal slice of that round: the moment you move, act, and react. Rounds keep going until combat ends.
Rogue
4 combatants are in initiative order. The Rogue is 3rd. She waits through turns 1 and 2, acts on turn 3, then waits again while turn 4 finishes. One round = 6 seconds. Then it repeats.
All characters
You can speak a few words on anyone's turn — it's all happening in the same 6 seconds. Coordinating a flanking call mid-round is always fine.
Movement
Move up to your Speed (usually 30ft) at any point during your turn. You can split it — move, act, move again — as long as the total doesn't exceed your speed.
Rogue (L2+ — Cunning Action)
Move 15ft to a shadow. Bonus Action: Hide (Cunning Action). Move 15ft closer. Attack from hiding — Sneak Attack applies.
L1: Move, Attack, move again — split movement around your action without any bonus action needed.
Difficult terrain
Costs 2ft of movement per 1ft travelled. Moving through an enemy's space is always difficult terrain.
Reaction
One response to a trigger. Usable once per round — on anyone's turn. Resets at the start of your turn. You must have an unused reaction to use one.
Rogue — Uncanny Dodge (L5+)
An archer hits the Rogue. She uses Uncanny Dodge as a reaction — halving the damage before it lands.
Others
Opportunity Attack (anyone, L1): enemy leaves your reach without Disengaging. Monk (L3): Deflect Missiles — reduce ranged damage; catch and throw back for 1 ki. Paladin: Shield of Faith is a concentration spell (action), not a reaction.
Action
The main thing you do on your turn. You get one action. Common options: Attack, Dash, Disengage, Dodge, Help, Hide, Ready, Use Object, Cast a Spell.
Rogue (L1)
Action: Attack. An ally is adjacent to the target — Sneak Attack (L1) adds 1d6 automatically. No hiding or advantage needed; a flanking ally is enough. Alternatively, attack from hiding (concealed on a previous turn) for advantage, which also triggers Sneak Attack.
Others
Paladin: Divine Smite is not an action — it triggers automatically on a hit. Monk: Extra Attack (L5) means two attacks within one Attack action.
Bonus Action
An extra action — but only if a feature grants it. You cannot use a bonus action unless something specifically says you can. Having one doesn't mean you must use it.
Rogue — Cunning Action (L2+)
Every turn, the Rogue can Dash, Disengage, or Hide as a bonus action. Disengage as a bonus action means she can move away from an enemy without provoking an opportunity attack — then use her full action to attack someone else.
Others
Paladin: no bonus action from base class at L1–2 (varies by oath). Monk (L2): Flurry of Blows — 1 ki for two bonus unarmed strikes after the Attack action.
Free Action
Minor things that cost nothing — a few words, dropping a held item, releasing a grapple, opening your hand. One object interaction (draw/stow a weapon) is also free per turn.
Rogue
While attacking: whispers "North door, now" to the Paladin (free). Drops a empty vial to free her off-hand (free). Draws a dagger as part of movement (free object interaction).
Note
A second object interaction in the same turn costs your action. Talking to an NPC to negotiate mid-combat is not free — that's an action (or at DM discretion).
Held Action (Ready)
Use your Action to Ready. Declare: (1) a trigger — "when X happens" — and (2) what you'll do. When the trigger fires, spend your Reaction to execute. If trigger never occurs, the action is lost.
Rogue (L1+)
"I ready an attack: if the guard rounds the corner, I strike." Trigger fires — reaction spent, she attacks. Sneak Attack (L1) applies if the conditions are met when the reaction fires — an ally adjacent, or advantage at that moment — not when she first declared the Ready.
Others
Readying a spell holds concentration from the moment you Ready it. If you take damage before the trigger fires, you may lose the spell. Readying Dash or Disengage is also valid.