In the field of maths, fractions, percentages and decimals represent the same thing - numbers less than 1. For KS3 Maths you should know the equivalences between fractions, decimals and percentages. All three types of numbers are used in day-to-day life and it's useful to be able to change from one to the other.
Any number can be written as a fraction, a percentage or a decimal. Let's look at a simple example: three quarters. As a fraction, the simplest way to write three quarters is as ^{3}⁄_{4}. Percentages write fractions as 100ths. One quarter of a hundred is 25 so three quarters as a percentage would be 25 x 3 = 75%. Decimals use tenths and hundredths. So, 75 hundredths (^{3}⁄_{4}) would be written as 0.75. It's all quite simple - once you've got the hang of it!
To help you understand how fractions work, take a look at the Fractions - Numerical Quantities article.