Gematria is the numerical valuation of words or phrases. For this reason, it is often called Numerology, but this term tends to imply a more New Age style computation and application of the valuation. Gematria tends to refer to the more traditional processes and purposes.
I don't know what sort of additional attributes are given to words or phrases under the seemingly endless forms of numerology out there, but according to The Magician's Companion, edited by Bill Whitcomb, the only "real" purpose for gematria is to identify pairs of words or phrases which, having valuated to the same result using some form of gematria, make a good subject of meditation or comtemplation, trying to see what the words might have in common, revealing relationships between the words that are otherwise not generally considered.
Gematria is often used to hide information, using a variety of techniques collectively called "Notorikon", from those not initiated to the system of Notorikon employed.
Although any valuation system which assigns numerical value to words and phrases is legitimately called gematria, there is pretty much one system of valuation which is considered standard for Hebrew and one for Greek. (I'll include the valuation tables to this page at another time.)
In English, there are multiple systems in use, but the most common include these:
1. A straightforward valuation of 1-26 for A-Z;
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
2. A rotation of the linear evaluation every 9 letters, which is a modified form of the AIK BEKAR system used with Hebrew gematria;
1 2 3 4 5 6 7 8 9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
3. E.·.Q.·., which uses the values 1-26 but in a permutation which skips to each 11th letter;
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 20 13 6 25 18 11 4 23 16 9 2 21 14 7 26 19 12 5 24 17 10 3 22 15 8
For more information on E.·.Q.·., read The Discovery of English Qaballa, by Carol Smith.
In Latin gematria, the most common system of valuation seems to be the Latin Qabala Simplex, called that because it is quite simple: The letters are simply assigned values from 1 to 22 in the order in which they occur.
My strongest source on the matter was "In The Continuum", Vol V, No. 1., which can be ordered from the Temple of Thelema.
The Latin Qabalah Simplex is simple:
A B C D E F G H I L M N O P Q R S T V X Y Z 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Then just add 'em up. No base 9 rotation like AIK BEKAR. Just simple addition.
INTERIORES ADVERTE I N T E R I O R E S 9+12+18 +5+16 +9+13+16 +5+17=120 A D V E R T E 1 +4+19 +5+16+18 +5=68 120+68=188 Using Roman Numerals, 188 is C (100) + L (50) + XXX (30) + V (5) + III (3) or CLXXXVIII
Hence, 'INTERIORES ADVERTE' is abbreviated 'I.A. 188' or 'CLXXXVIII'.