Simply put, a decimal is a number with a period in it. Here are some examples of decimals:
The decimal has two parts. The number to the left of the period (or decimal point, as it's called) is the whole number. The number to the right of the decimal point is the fractional number, the part that's smaller than one.
Let's look at dollars and cents, because that's how I learned about decimals and fractions. In the third example above, 5.25 would be five dollars and 25 cents -- 5 being the whole number, and 25 being the fractional number.