The Ninth House represents the higher mind. It is associated with foreign travel, religion, higher education, certain areas of the legal system, and the social mores of society. More than anything, the ninth house calls you to open your mind to your highest spiritual truth to learn who you are, beyond society's labels.
In general, the ninth house represents your in-laws and is one of the places to look for grandchildren.
Because this house is naturally ruled by Sagittarius, it determines the level to which you express integrity and honesty, as well as your experiences with education. Gemini on the ninth house cusp may indicate someone who is constantly going back to college to get another degree. Aries in the ninth house may be compelled to preach to you about how things are, even if they're wrong!
Natal planets in the in the ninth house add flavor to the philosophical nature. Saturn in the ninth may indicate a conservative approach to religion and philosophy. Uranus in the ninth may indicate a person who leaves the family religion or works for philanthropic causes.