Wrinkles on the forehead most commonly appear in two ways:

As vertical lines just above the nose, often referred to as “the elevens”; or horizontal rows across the forehead. The “elevens”, or glabellar lines, result from squinting, scowling, worrying or just plain thinking.
The horizontal lines may result from normal animation while talking, or from holding the brows up with the primary muscle of the forehead (the frontalis) during times of stress, or just naturally. These lines are primarily the result of continual muscle contraction and can become very deep over time.