'Percent' means 'out of 100'. A percentage is a fraction of 100.
In some questions you are given the cost price and the selling prices and have to find the percentage increase or decrease. This means you need to find one amount as a percentage of another. You form a fraction from the two amounts and multiply this by 100. Have a look at this website, and try this activity after reading. Check if you understood doing this test.
To study in depth, have a look here. And check what you learned with this test.
And last but not least, see this problems here to finish your study.