Gematria is a system of recognizing a correspondence between the ten sefirot, or fires of God, and the twenty two letters in the Hebrew alphabet. This system is elaborated in many mystical Jewish writings such as the Zohar.
One example of Gematria is that there are twenty-two solid figures that are composed of regular polygons. There are five Platonic solids, two Kepler solids, two Poinsot solids, and thirteen Archimedean solids. Since there are twenty-two letters in the Hebrew alphabet (aleph-beth), a correspondence is possible between these two facts. The art of Gematria is knowing which solid is associated with which letter.
Another example is that of Hebrew numerals. Although there are twenty-two letters, there are twenty-seven numerals necessary to sum each number up to one thousand. (one through nine, ten through ninety, one hundred through nine hundred) The Hebrew numeric system notes that the missing final five letters of the numeral system match exactly with the five 'sofeet' forms of the Hebrew letters, which are alternate forms of particular letters only used when that letter is the last consonant in a hebrew word.
Another use is that words which have the same numerical value, share the same qualities, and reveal still other aspects of the Divine.