Sigle vs. acronyme
In French, both acronyme and sigle are used to describe abbreviations formed from the first letters of multiple words. The key difference between the two lies entirely in how they are pronounced out loud. A sigle is pronounced letter by letter, whereas an acronyme is pronounced fluidly as if it were a standard word.
Sigle
A sigle is an abbreviation formed from the initial letters of several words that is spoken letter by letter, similar to an initialism in English.
Le professeur a expliqué que chaque lettre de ce sigle a une signification importante.
(The professor explained that every letter of this initialism has an important meaning.)
Il est parfois difficile de retenir un sigle quand il est composé de nombreuses lettres.
(It is sometimes difficult to remember an initialism when it is made up of many letters.)
Le sigle TGV désigne les trains à grande vitesse qui traversent la France.
(The initialism TGV designates the high-speed trains that travel across France.)
Elle a écrit un article sur la façon dont chaque institution gouvernementale utilise un sigle différent.
(She wrote an article about how each government institution uses a different initialism.)
Acronyme
An acronyme is a specific type of abbreviation made from the first letters of other words, but it is pronounced as a single, fluid word rather than spelling out the letters.
Le mot laser est en fait un acronyme qui provient de l'anglais.
(The word laser is actually an acronym that comes from English.)
Pour faciliter la communication, l'équipe a créé un acronyme amusant pour son nouveau produit.
(To facilitate communication, the team created a fun acronym for their new product.)
Cet acronyme se prononce tellement bien qu'on oublie souvent qu'il s'agit d'une abréviation.
(This acronym is pronounced so well that people often forget it is an abbreviation.)
Il faut s'assurer que l'acronyme choisi ne ressemble pas à un mot offensant.
(You must make sure that the chosen acronym does not sound like an offensive word.)
Summary
To summarize, while both words refer to abbreviations made of initial letters, the distinction is phonetic. If you spell out the letters one by one, you are using a sigle. If you pronounce the letters together to form a brand new spoken word, you are using an acronyme.