Levels are basic. Curves give you surgical precision over color and tone. This is how professionals color grade.
Access: Layer → New Adjustment Layer → Curves
The RGB Curve: Drag middle upward = brighten midtones. Drag downward = darken. S-curve = increase contrast.
Individual Color Channels: Switch from RGB to Red/Green/Blue channels. Pull red curve up = add warmth. Pull blue curve down in shadows = add warmth to shadows only.
Cinema Look: Lift shadows (black point) slightly, pull down highlights slightly. Creates that matte film look.
Master Curves = Master Photoshop!