Numerology is an interesting little branch that adds up things like the letters in your name and date of birth before giving you an overview and description. It has 11 numbers: 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 22. You can read the keywords for each number here.

Numerology depends on a strange method of adding called "reduction". In example, if you add up all the numbers in your name and you get 78:
7 + 8 = 15
1 + 5 = 6
Your number is therefor 6!

Numerology is a lot deeper than a bit of weird math, so feel free to research it further if it catches your interest.

Life Path Number
Who a person is at birth and the native traits that will guide them through life.
Name Number and Destiny Number
Breaking down the birth/given name.
Inner Dreams and Personality Number
The sum of the consonants in a name relates to a secret dream, inner desires, or perhaps fantasies. Also associated with personality and first impressions.
Soul Urge and Heart's Desire Number
The vowels of the birth/given name determine inner cravings, likes and dislikes, private feelings, and most valued beliefs.
Tarot Birth Cards
The individual numbers in the date of birth added up to find the corresponding Tarot cards.

Recommended Websites:
Michael McClain, Astrology and Numerology
Avia Venefica, What's Your Sign - Numerology