A lot of stores have configurable products. It may presented as different color or size of product on product page. The functionality what magento have by default this is change of price when dropdown option is selected for configurable product. Basically each configurable product contains few associated simple products. And in case you want change some information on product page when customer select option its impossible with default functionality, and you will have to write code to add this feature into magento. However once of extensions Configurable Image Switcher allows you change main image of associated simple product for selected option. Using this extension main image of your configurable product will change depend on selected option in dropdown, it will give customers ability to see exactly product which they are looking like color or size.