The Options Tab allows you to change and customize how each tool will work in Photoshop. The Options Tab itself will change depending on which tool you have selected. Things such as size of the cursor, add or subtract from the selection, and the style are common features to many tools.
This shot shows the rectangular marquee as the selected tool, and the options that you get.
This shot shows the crop tool as the selected tool, and the different set of options you get.
Make sure you pay attention to the different options available for each tool, and play around with them to find what works for you and your project.