Acronyms are a kind of abbreviations. Acronym word comes from Greek, meaning heads of names.
An acronym is a word formed from each of the first letters of a phrase. Occasionally, for special reasons, the second letter of a word is used. Not all acronyms are pronounceable.
NATO = North Atlantic Treaty Organization
Laser = Light Amplification by Stimulated Emission of Radiation
Radar = Radio Detection and Ranging
FAQ = Frequently Asked Question
BBC = British Broadcasting Corporation
Modem = Modulator/Demodulator
You can find abbreviations and acronyms with this site acronymfinder.com.
Internet Acronyms Dictionary located at gaarde.org/acronyms has an easy to browse database of acronyms sorted by first letter.
Another site dedicated to acronyms is acronyms.silmaril.ie.