The terms number and numeral have different meanings that tend to be confused. Although numbers and numerals have no real difference when you speak them, trouble arises when you attempt to write each of them. There are certain rules that apply to each, and understanding them will make you a stronger writer.
Basic Difference

A numeral is the way we express a number in writing. This can be with a word, a written digit and Roman numerals. For example, the number two can be expressed with the word "two," the numeral "2" or the Roman numeral "II."
Numbers

Numbers are an imaginary count of items. They are abstract, like a person's name. There are several numerals to express the same number.
Writing Digits

When writing digits, the common form of numerals, write the word for the number when it is from 1 to 9. Write the digit to express numbers higher than 10. When beginning a sentence, always write the word for the number rather than using the digits.
Special Numbers, Numerals

Some numbers, such as pi and those that are negative, are special in that they are somewhat abstract. The number pi is the ratio of a circle's circumference to its diameter. Although this is easy to understand, it is impossible to articulate fully as a numeral. Most of the time, you use 3.14 as the numeral for pi, but its decimal places are infinite. Negative numbers are easy to write by putting the negative sign before the digit, but they are tough to get your mind around when thinking abstractly about them.