"Digits" are the symbols or characters used to represent a number visually. A number like five contains one digit, whereas a number like 555 contains three digits. This is easily seen when they are written as numerals: 5 versus 555.
Digits are distinguished from numerals in that numerals are the generalized symbols, such as 2, 3 or 4, that are combined to represent numbers, whereas digits are these symbols as used in a specific number. The symbols 1, 2, 3 and so on are unique to the Arabic numeral system; other systems, such as the Roman numeral or Hebrew numeral system, use letters from their respective alphabets. The same number in different numbering systems may have a different quantity of digits. For example, in base 10, five has one digit, but in binary the number five has three digits.