Acronimo vs. sigla
Both acronimo and sigla refer to abbreviations formed from the initial letters of other words, but they differ in how they are pronounced. A sigla is a broad term for an initialism that is typically pronounced letter by letter. An acronimo is a specific type of sigla that is pronounced fluidly as a single, new word.
Acronimo
An acronimo is a word formed from the initial letters or syllables of a phrase, and it is pronounced exactly as it is written, like a normal word.
Il professore ci ha chiesto di spiegare questo acronimo.
(The professor asked us to explain this acronym.)
Il dizionario include un'appendice dedicata a ogni acronimo moderno.
(The dictionary includes an appendix dedicated to every modern acronym.)
Spesso usiamo un acronimo senza conoscerne il significato completo.
(We often use an acronym without knowing its full meaning.)
Hanno scelto un acronimo molto orecchiabile per il nuovo progetto.
(They chose a very catchy acronym for the new project.)
È più facile ricordare il nome di un'azienda se usa un acronimo.
(It is easier to remember a company's name if it uses an acronym.)
Sigla
A sigla is an initialism formed by the first letters of a sequence of words, usually pronounced letter by letter, though it serves as a general term for organizational abbreviations.
Non riesco a capire cosa significhi questa sigla sul documento.
(I cannot understand what this initialism on the document means.)
Per brevità, il nome del sindacato viene ridotto a una sigla.
(For brevity, the name of the trade union is reduced to an initialism.)
Ogni partito politico è rappresentato da una sigla.
(Every political party is represented by an initialism.)
Dobbiamo inserire la sigla della provincia nell'indirizzo di spedizione.
(We must insert the province's initialism in the shipping address.)
Il giornalista ha usato una sigla che nessuno nel pubblico conosceva.
(The journalist used an initialism that no one in the audience knew.)
Summary
In short, every acronimo is technically a sigla, but not every sigla is an acronimo. You should use sigla for general abbreviations that are spelled out letter by letter, and acronimo when those combined letters can be spoken aloud as a brand new word.