In addition to using the Emoji Viewer to write special characters, or holding a key to add accents, you can also learn some special keyboard combinations using the Keyboard Viewer. These allow you to type some symbols such as degrees, pi, the Apple symbol and accent marks using standard keystrokes using the Option and Shift keys. Keyboard Viewer lets you see exactly which key combinations match which characters. This depends on the type of keyboard you own.
Check out how to use hidden characters on your Mac keyboard on YouTube for closed captions and more options.