Each letter in the Hebrew alphabet has a numerical value.
The first ten letters א to י have the values 1 to 10.
The next 9 letters כ to ק have values 20, 30, ... 100.
The last ת, ש, ר have values 200, 300, and 400.

אבגדהו זחטי
12345678910

כלמנסע פצק
20304050607080 90100

רשת
200300400

The numbers 15 and 16 should not be represented by the
combinations "יה" and "יו" as these have religious significance,
and so are expressed as טו = 9+6 and טז = 9+7.

Numbers above one thousand — ת”תר — are represented
as: First, the thousands count, followed by the remainder.
When the thousands count is understood from context, it may be skipped.
E.g. 5772 is ה’ תשע”ב
that is: 5 thousand, 400 + 300 + 70 + 2.

For a more detailed explanation, see http://en.wikipedia.org/wiki/Hebrew_numerals