A decimal is a way to express fractions. A whole number is placed to the left of the decimal point (also called period or full stop). Digits to the right of the decimal indicate a fraction of a whole, as in tenths, hundredths, thousandths and so on.
The most frequent use of decimals is with money. $1.23 means 1 dollar and 23 cents, where each cent is 1/100 of a dollar. Half a dollar is $0.50, meaning 50/100 of a dollar. An American "quarter" is .25 of a dollar, equal to 25/100 of a dollar or 25 cents.
When writing decimals less than 1, a 0 does not have to be placed on the left side of the decimal point. While not necessary in general usage, proper usage of zeros is essential in chemistry and physics where significant figures apply. A dash over the final digit (or digits) indicates that digit repeats infinitely, or without ending. Such cases are referred to as repeating decimals.