Any tool in the Toolbar can be assigned a letter as a keyboard shortcut in order to access it more quickly (type that letter, get that tool). To view the keyboard shortcut assigned to a tool, hover your cursor over the tool to display the tool tip or, click and hold on a tool to view the list of tools (the shortcut is listed to the right of the tool name).
To save screen real estate, Photoshop nests similar tools together in the Toolbar. To quickly cycle through nested tools, Option -click (Mac) | Alt -click (Win)on the tool
Photoshop also assigns the same shortcut to multiple, similar, tools. For example the Lasso, Polygonal, and Magnetic Lasso tools all have the shortcut “L” assigned to them. To cycle through the tools using the keyboard shortcut, add the Shit key (Shift + L for example). If you prefer to cycle through tools that have the same shortcut without using the Shift key, choose Preferences > Tools and disable “Use Shift Key for Tool Switch”.
If a tool doesn’t have a shortcut key assigned (or has one that you don’t like), choose Edit > Toolbar and use the editor to assign your own custom shortcuts by clicking in the empty space to the right of the name of the tool. To remove a shortcut, select it in the editor and tap the delete key.
The Toolbar Editor also has options to hide tools as well as regroup them. This video (The Start and Recent Files Workspaces and Customizable Toolbar in Photoshop CC) demonstrates how.
There are additional options for assigning keyboard shortcuts to Tools using Edit > Keyboard Shortcuts. Select Shortcuts For: Tools, scroll down to the bottom of the list, and assign your own shortcuts to commands such as displaying the Foreground or Background Color Picker, Loadingthe Mixer Brush, Sharpening Erodible Tips and more.