The selection tools are what you will use to select portions of an image to edit. While each of these tools are for selection, they differ in how they go about selecting, and their levels of sophistication vary.
Each icon in the Tools Panel also houses other tools. To see these tools, click and hold any one of these icons and a list will appear displaying the alternate options.
Additionally, each tool has varying features that are accessed in the "Options Menu", which are discussed in the "Options Tab".
A common procedure after selecting a portion of an image is to crop that portion out, usually to add to another image or because the cropped image looks nicer than the original. These tutorials will help you learn what you can do with cropping.