MP3 and AAC are both lossy file configuration for audio recording. AAC stands for Advanced Audio Coding. The critical difference is in the quality of the record after it is encoded with either AAC or MP3. AAC has corrected a lot of shortcomings of MP3 when encoding sound files, in short AAC is by and mostly better than MP3, more so at lower bitrates.
The MP3 format has more significant support, especially in portable devices, compared to AAC. MP3 is already a well-known audio codec that is used mostly in mobile media players, due to the significant decrease in file size it offers.