An initialism is an abbreviation formed by using the first letters, or initials, of a series of words, for example "BBC", or "IBM". The term initialism is often used by those who make a sharp distinction between an initialism and an acronym; they reserve the term acronym for cases when the letters form a pronounceable word, like "NATO" or "AIDS", and use the term initialism when they do not, being pronounced instead by sounding out the name of each constituent letter. (Thus, by this definition, BBC ("Bee Bee See") is an initialism, while NATO ("Naytow") is an acronym.) Others do not make this distinction, and use the terms interchangeably (though the term acronym is used much more frequently in this case).
In some cases, an initialism has been turned into a name because the meaning of the initials has been superseded. The YMCA is now just a name and is no longer officially short for Young Men's Christian Association. Likewise the letters making up the name of the SAT college entrance test, that is, "the College Boards", no longer officially stand for anything.
The official "full" form of an initialism -- what the initialism stands for -- may also be changed without revising the initialism. SADD, for instance, originally Students Against Drunk Driving, changed the full form of its name to Students Against Destructive Decisions. YM once stood for Young & Modern, but now stands for simply Your Magazine.