Preferences are settings that apply to the application as a whole, such as which ruler units are used, or if channels display in color. Most preference changes take effect immediately; a few take effect upon re-launching (we’ve noted those exceptions). All are saved when you exit/quit Photoshop.
To access the General Preferences dialog box the fast-and-easy way, press Ctrl-K/Cmd-K. From there, you can cycle through the various panes using the shortcuts listed on the pop-up menu . You can also use the Edit (Photoshop, in OS X) > Preferences submenu to access the preferences dialog boxes.