Decimal

Decimal, also called denary, is the base 10 numeral system, which uses symbols 0-9 (called digits).

Decimal is the predominant numeral system used by humans, though some cultures do or did use other number systems. This is almost certainly because humans have ten fingers; digit is also the anatomical term referring to fingers and toes.

Computers use other number systems and so, sometimes, do people using computers. Binary, octal and hexadecimal are common, because they are easy to implement on a computer. Humans' use of decimal gave rise to binary-coded decimal in the context of computers.

Fractions

Ten is the product of the first and third prime number and is one greater than the square of the second prime number. This leads to plenty of simple decimal fractions:

1/2 = 0.5
1/3 = 0.3333 repeating
1/4 = 0.25
1/5 = 0.2
1/6 = 0.16666 repeating
1/8 = 0.125
1/9 = 0.1111 repeating