A diacritic[1] is a mark put above, below, through or on a letter. Some examples of diacritics are an acute accent or a grave accent. The word comes from the Greek word διακριτικός (transl. diakritikós, 'distinguishing').
Usually, it affects the way the word is said (pronounced). Most diacritics concern pronunciation because most alphabets do not describe the sounds of words exactly. Diacritics are rare in English, but common in many other languages.