Firstly, there is nothing wrong with stretch marks and most women will notice stretch marks on their skin at some point during their life - they are extremely common.
They generally appear as different coloured parallel lines on the surface of your skin and can develop anywhere on the body, but are most common on the stomach, arms and thighs. They are usually red or purple at first, before fading to more of a white colour and in some cases you may even be able to feel a slight indentation when you touch them.
As the name suggests, stretch marks are a result of the skin stretching rapidly. This causes the middle layer of skin to tear slightly and the deeper layers to show through. The reason that stretch marks are often red or purple in colour is because these tears allow the blood vessels to show through. Hence, red or purple stretch marks tend to be stretch marks that are newer, whereas silver or white stretch marks tend to be older stretch marks that have faded.