Switching apps. Cmd+Tab cycles through open applications in order of recent use; hold Cmd and tap Tab to move forward, and add Shift to move backward. Cmd+backtick cycles through the open windows of the current app.
Hide, minimize, quit. Cmd+H hides the current app (removing it from view without quitting), Cmd+M minimizes the front window to the Dock, Cmd+W closes the front window, and Cmd+Q quits the app entirely. Cmd+Option+Esc opens Force Quit for apps that stop responding.
Mission Control and Spaces. Control+Up (or F3) shows Mission Control with every open window, and Control+Down shows all windows of the current app. Control+Left and Control+Right move between Spaces, and Control plus a number jumps to a numbered Space once that option is enabled under System Settings, Keyboard, Keyboard Shortcuts, Mission Control. Stage Manager, available in Tahoe, groups windows by app along the left edge; its toggle is in the table above.
Full screen. Cmd+Control+F toggles full-screen mode for the current window.