The tool that's initially displayed in each spot in the toolbar is known as the default tool. For example, the Rectangular Marquee Tool is the default tool for the second spot from the top. But Photoshop won't always display the default tool. Instead, it will display the last tool you selected.
Notice that after choosing the Elliptical Marquee Tool from the fly-out menu, the Rectangular Marquee Tool is no longer displayed in the toolbar. The Elliptical Marquee Tool has taken its place:
