AAC is a sound encoded file format developed by Fraunhofer IIS, AT&T, Dolby and Sony. Its name stands for Advanced Audio Coding. The AAC format is an industry standard and was created as an improved alternative to the most common MP3 audio file format. AAC files do have slightly improved features compared to MP3s. Let's say this format provides the opportunity for improved processing of frequencies above 16 kHz. At the same time, the sound quality of AAC files is closer to the original sound source.
FLAC is a free cross-platform audio codec format created in 2001 by Josh Colson and Xiph.Org. The name of the format is an abbreviation of the English phrase Free Lossless Audio Codec. When compressing a file into the FLAC format, it is 50-60% of the original file without loss of sound quality (especially when working with voice recordings). When unpacking such a file, absolute originality is respected. Since the format is free and open, it is supported by almost any software, it is perfect for archiving files and for regular listening to digital recordings.