Key planets generally refer to the Sun and Moon in the chart, along with the ruler of the sign on the Ascendant. Not to diminish the importance of the other heavenly bodies, but these planets are the most personal points in the entire chart. Because of the personal interfacing, these planets reflect the most sensitive side of your being.
The Sun is your purpose in life. It's what you have come here on Earth in the incarnation to do and express. If you are not expressing what your Sun needs then you will find yourself unhappy and consequently very unfulfilled.
So often, especially younger people in their teens and early 20s, will ask "why am I here? What's my purpose?" That answer is found in their Sun and it's Zodiac Sign and the house placement in the chart wheel. Aspects to the Sun are indications if the individual will have an easy time fulfilling that purpose or not. Remember, the aspects color how easy or difficult the expression is and the Sun is all about self expression.
The Moon is your emotional receptor. It holds your past memories and we can almost say your ancestral ties. The Sun is born to radiate, and the Moon to be receptive. The lunar principle is to collect and filter through the feelings. It's your sensory device and your inner radar, which is most beneficial if you take the time to listen and feel her responses in your gut.
The Moon shows how you relate emotionally to yourself, nurture yourself, and emote. Again, the aspects the Moon makes in your chart will be very telltale about how you address emotional situations.
The Ascendant is what people see in you at a first glance. It's what you project of yourself to others. The ruling planet by sign, house, and aspect will give more information about your modus operandi - basic elements of your character and attitudes toward self and relationships.