They are small marks added to letters to indicate a change in pronunciation. Examples of diacritics include:
* Acute accent: á (as in "café")
* Grave accent: à (as in "fútbol")
* Circumflex accent: â (as in "cliché")
* Tilde: ñ (as in "señor")
* Umlaut: ö (as in "Über")
* Cedilla: ç (as in "garçon")
There are many other diacritics, and their specific use varies depending on the language.