The 26 alphabets of French are used and pronounced differently by using 5 types of accents. The little marks that you see above or below certain alphabets in French words are called accents. These accents are mainly used to change the pronunciation of a vowel and to differentiate between similarly spelled words

The 5 accents are:

L’accent Grave - grave accent (à, è, ù):

Over a or u, used only to distinguish homophones: à (”to”) vs. a (”has”), ou (”or”) vs. où (”where”). Over an e, indicates the sound /?/.

L’accent aigu - acute accent (é)

Over an e, indicates the ai sound in such words as English hay or neigh.

circumflex (â, ê, î, ô, û)

Over an a, e or o, indicates the sound /?/, /?/ or /o/, respectively (the distinction a /a/ vs. â /?/ tends to disappear in many dialects). Most often indicates the historical deletion of an adjacent letter (usually an s or a vowel) like château for castel, fête for feste, sûr for seur, dîner for disner etc. By extension, it has also come to be used to distinguish homophones: du (”of the”) vs. dû (past participle of devoir “to owe”; note that dû is in fact written thus because of a dropped e: deu).

diaeresis or tréma (ë, ï, ü)

Indicates that a vowel is to be pronounced separately from the preceding one: naïve, Noël. Diaeresis on y only occurs in some proper names (such as l’Haÿ-les-Roses) and in modern editions of old French texts. The diaresis on ü appears only in one non proper name: Capharnaüm. Nevertheless, since the 1990 orthographic rectifications (which are not applied at all by most French people), the diaeresis in words containing guë (such as aiguë or ciguë) may be moved onto the u: aigüe, cigüe.

cedilla (ç)

Indicates that an etymological c is pronounced /s/ when it would otherwise be pronounced /k/. Thus je lance “I throw” (with c = [s] before e), je lançais “I was throwing” (c would be pronounced [k] before a without the cedilla).

Related:

French Words Used in English

Names of Colors in French