Using the "Compare" button, you can store products in the toolbar on the bottom of your screen to compare later.
Click on an item to toggle between "selecting" and "deselecting" it. Products are selected by default. When you hover over a product on the toolbar, you will have the options of viewing the item or removing it.
You must have at least two products selected in the toolbar to compare them. Once you do, click on the "Compare Selected Items" button on the toolbar.