There are four fundamental forces involved and acting on the airplane at all times in flight.
They are:
THRUST AND DRAG (THRUST opposes DRAG)
LIFT AND WEIGHT (LIFT opposes WEIGHT)
Watch the video above for a more thorough explanation.
Remember that:
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!