In the history of mysticism, numbers have long been held as sacred.  There is a sense that they exist "a priori" in a Kantian sense, I.e., that they exist whether or not there are any humans or other sentient creatures around to count them.  Some have suggested that they might be the only "true" universal alphabet.  It is hard to imagine that they might be explicitly human developments -- a concept which lends itself readily to the notion that they are divine.
Pythagoras believed that numbers were literally the "stuff" out of which the things of universe were made.  By this of course he meant integers, or ratios of two integers -- rational numbers.  What, after all, cannot be described with numbers?  To describe is to name, and to name is to have power over something; this lesson is codified prominently in the Biblical book of Genesis, and is a powerful formula in magick. Therefore numbers "ruled" the universe.  How vexing it was, then, for Pythagoras when he discovered that there were irrational numbers that could not be described with other numbers. Numbers important to geometry, which could not just be cast aside, turned out to be irrational -- pi, the square roots of 2, 3, and 5, and so on.
Pythagoras developed notions about numbers that exist to this day.  In his theory of numbers, 1 was considered the ancestor of the other numbers, and, in a sense, not properly a number as the others were.  Even numbers were "female" while odd numbers were "male."  He also developed the concept of the prime number, a number that cannot be evenly divided by any other number.  Another concept introduced by Pythagoras was that of the perfect number: this is a number such that its multiplicative factors (except itself) also add up to give the number.  The first perfect number is 6:  1, 2, 3, and 6 are the factors of 6 (since 6 = 1x6 = 2x3), and 1+2+3 = 6.  Relationships between two numbers were important to Pythagoras; for example, he defined friendly numbers which had a relationship such as this, between 220 and 284:
284 = 1x284 = 2x142 = 4x71
220 = 1x220 = 2x110 = 4x55 = 5x44 = 10x22 = 11x20
Adding the factors as though they are perfect numbers (all but the number itself), we find that
1+2+4+71+142 = 220
1+2+4+5+10+11+<WBR>20+22+44+<WBR>55+110 = 284
He also defined functions for the various plane figures: triangles, squares, etc.:
Number triangle square
1 1 1
2 3 4
3 6 9
4 10 16
5 15 25
6 21 36
7 28 49
8 36 64
9 45 81
10 55 100
The square numbers, as you can see, are still used today.
More on the Web:
Excerpts from Plato and Archimedes on Pythagoreans
Pythagoras
Qabalistic meanings of the numbers
Number Sphere / Path Hebrew Letter Name Gematric Value Tarot Attribution
1 Kether Crown Aces
2 Chokmah Wisdom Twos, Kings Zodiac
3 Binah Understanding Threes, Queens Saturn
4 Chesed Mercy Fours Jupiter
5 Geburah Strength Fives Mars
6 Tiphareth Beauty Sixes, Princes Sun
7 Netzakh Victory Sevens Venus
8 Hod Splendor Eights Mercury
9 Yesod Foundation Nines Moon
10 Malkuth Kingdom Tens, Princesses Earth
11 0 Aleph Ox 1 Fool Air
12 1 Beth House 2 Magician Mercury
13 2 Gimel Camel 3 Priestess Moon
14 3 Daleth Door 4 Empress Venus
15 4 Heh Window 5 Emperor Aries
16 5 Vau Nail 6 Hierophant Taurus
17 6 Zain Sword 7 Lovers Gemini
18 7 Cheth Fence 8 Chariot Cancer
19 8 Teth Snake 9 Strength Leo
20 9 Yod Hand 10 Hermit Virgo
21 10 Kaph Fist 20, 500 Wheel Jupiter
22 11 Lamed Ox Goad 30 Justice Libra
23 12 Mem Water 40, 600 Hanged Man Water
24 13 Nun Fish 50, 700 Death Scorpio
25 14 Samekh Prop 60 Temperance Sagittarius
26 15 Ayin Eye 70 Devil Capricorn
27 16 Peh Mouth 80, 800 Tower Mars
28 17 Tzaddi Fish Hook 90, 900 Star Aquarius
29 18 Qoph Ear 100 Moon Pisces
30 19 Resh Head 200 Sun Sun
31 20 Shin Tooth 300 Judgment Fire
32 21 Tau Cross 400 Universe Saturn

Another way to look at the numbers qabalistically is to list what they count:
3, Mother letters
4, Elements, Directions, Worlds
7, Planets (traditionally); Double letters
10, Sephiroth
12, Signs of the Zodiac; Simple letters
22, Paths
28, Mansions of the Moon; Kings+Seniors (Enochian)
30, Aethyrs (Enochian)
36, Decans of the Zodiac
72, Quinances of the Zodiac; Shemhamphoresh Angels
78, Cards in the Tarot
91, Parts of the Earth (Enochian)
216, Letters in the Shemhamphoresh
231, Gates to Understanding

Many sets of symbols in magick are based on these numbers.  For example, each decan of the Zodiac has an angel, and each has a demon.  In many ways these numbers are magickal "codes" or "keys" of use in raveling, and unraveling, the symbolism used in ritual.  (Another key, of course, is gematria, which is discussed elsewhere.)