Certain cards in the tarot naturally form into groups. These cards have unique meanings, but they also have a common identity with the other cards in their group. The two largest subgroups are the major and minor arcanas. The terms major and minor reflect the relative weights of these two groups.
A major arcana card represents an energy that is deep, strong, decisive or long-term. When a major arcana card appears in a reading, you have tapped into a powerful energy in some area of your life. The minor arcana cards do not carry the same weight, but they are still important. They chart the ups and downs of daily life and register changes in feelings and thoughts. These dramas are gripping while they occur, but they pass with time as new concerns take their place.
Compare the interpretation of two cards with similar meanings, but different weights - the Hermit (major) and the Eight of Cups (minor). The Hermit is the archetypal symbol of one who seeks truth and deeper meaning. He stands for the impulse to renounce superficial pleasures to seek understanding within. In a reading, the Hermit could hint that you are feeling a strong urge to find answers even if it means giving up aspects of your current way of life. This is not a passing fancy, but a major desire that could last for some time.
With the Eight of Cups, your interpretation might be much the same, but, as a minor arcana card, this card implies your search does not have the same force. Maybe you've been a little bored at work. Some days you feel like chucking everything to go make hats on the beach, but you don't really mean it. You are seeking, but the urge is not yet a heartfelt desire.
Let's say you have been happily married for many years, but suddenly find yourself attracted to an acquaintance.
You consult the tarot and draw the Two of Cups.
One of the meanings of this card is attraction - being drawn toward a person, place or idea that is pleasing. As a minor arcana card, the Two of Cups tells you your attraction is probably based on surface elements such as common interests or sexual desire. The feelings are strong, but they're mainly coming from the give-and-take of everyday adventure - for now.
If you had drawn the Lovers instead, you would have to give this attraction greater weight. As a major arcana card, the Lovers implies this relationship is not simple. The attraction is coming from a deeper place. There are elements that go beyond the casual and require more understanding.
You could draw a minor arcana card in one reading and then draw a major arcana card about the same subject in a later reading. What started as minor has become more important over time. Similarly, a major matter can fade and lose its urgency as your life changes. You can assume that a major arcana card brings greater energy to whatever area it represents. Give that aspect extra consideration and take advantage of the power building there.