AAC and MP3 are both lossy audio compression formats. AAC is the default format used by iTunes. It produces better audio quality than MP3 for a given data rate (e.g., 128 kbps) and is playable in almost all software that can play MP3 files.
CAUTION: If your music contains sounds copyrighted by other people, and you don’t have permission to use those sounds, then think twice about uploading your music to a web site. That counts as distribution, and the copyright holder may hold you accountable for your use of their sound.