AAC Audio File Format. Advanced Audio Coding.
Development of methods for data compression and psychoacoustics gradually led to the fact that the standard MP3 became an obstacle for the implementation of new ideas in the encoding process. As a result, by 1997 the Fraunhofer Institute (Fraunhofer IIS), which in the early 90's has created an MP3, as well as companies Dolby, AT & T, Sony and Nokia - have developed a new method of compressing audio - Advanced Audio Coding (AAC), which entered the MPEG-2 and MPEG-4.
The main differences from MP3 are:
Due to these features, AAC standard is able to achieve a more flexible, efficient and higher quality audio coding. Due to widespread of MP3 format, AAC standard is still not comparable to the MP3 gained popularity. However, AAC is the basic format of iTunes Store, iPods iPod, iTunes, phone, iPhone, PlayStation 3, Nintendo Wii and a digital radio DAB / DRM. Raw AAC files not so popular. In most cases, AAC-encoded audio distributed as MP4 or M4A audio files.
Here you can find some tutorials how to convert AAC files: