This can be disputed. I like this distinction. I think it's a useful distinction. I use it myself. But I think it is pretty much a lost cause. I'm going to check a dictionary now, and I expect the dictionary to report that the word "acronym" CAN mean an initialism. Here we go.
American Heritage dictionary:
1. A word formed by combining the initial letters of a multipart name, such as NATO from North Atlantic Treaty Organization or by combining the initial letters or parts of a series of words, such as radar from radio detecting and ranging.
2. Usage Problem: An initialism.
Usage Note: In strict usage, the term "acronym" refers to a word made from the initial letters or parts of other words, such as "sonar" from "so(und) na(vigation and) r(anging)." The distinguishing feature of an acronym is that it is pronounced as if it were a single word, in the manner of "NATO" and "NASA." Acronyms are often distinguished from initialisms like "FBI" and "NIH," whose individual letters are pronounced as separate syllables. While observing this distinction has some virtue in precision, it may be lost on many people, for whom the term "acronym" refers to both kinds of abbreviations.