The decimal system is a way of managing numbers that has ten as its base. It is sometimes called the base ten or denary numeral system. The word "decimal" is also used instead of the word "period" to point out the dot (also called the radix point) that is sometimes used to separate the positions of the numbers in this system. This is the common every-day numbering system that people use in most of the world. Computers use binary or base 2. The length of a number (how many digits it takes to write the number) depends upon the base.
Decimal notation is the writing of numbers in the base-ten system, which uses various symbols (called digits) for no more than ten distinct values (0, 1, 2, 3, 4, 5, 6, 7, 8 and 9) to represent any numbers, no matter how large. These digits are often used with a decimal separator which indicates the start of a fractional part, and with one of the sign symbols '+' (positive) or '-' (negative) in front of the numerals to indicate sign.
Adapted from: Wikipedia "Decimal"