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!