There are four fundamental forces involved and acting on the airplane at all times in flight.
THRUST AND DRAG (THRUST opposes DRAG)
LIFT AND WEIGHT (LIFT opposes WEIGHT)
Watch the video above for a more thorough explanation.
IN UNACCELERATED FLIGHT, ALL FORCES ARE IN BALANCE (or equal). Even if the airplane is climbing or descending. The only thing that makes the forces unequal to each other is ACCELERATION!