How AAC Improves on MP3
AAC is a lossy codec like MP3, but with a more efficient toolbox: pure MDCT coding without the hybrid filter bank MP3 inherited, more block-size flexibility for handling transients, and better handling of frequencies above 16 kHz. In listening tests, AAC at 128 kbps generally matches or beats MP3 at 160-192 kbps. The MP3 vs AAC comparison covers this in depth.
Technical Characteristics
The common profile is AAC-LC (Low Complexity), used at 128-256 kbps for music; HE-AAC and HE-AACv2 add spectral band replication and parametric stereo for efficient streaming down to 32-64 kbps. AAC supports sample rates from 8 to 96 kHz and up to 48 channels. It is usually delivered inside an MPEG-4 container- .m4a for audio-only files, .mp4 for video- Or as a raw .aac stream (ADTS). Like MP3, AAC can be encoded CBR or VBR, and mature encoders- Apple's in particular- Are considered the quality reference for the format.
When to Use AAC
AAC is a great default for the Apple ecosystem, for streaming delivery, and whenever you want slightly better quality than MP3 at equal file size. iTunes rips and Apple Music downloads are AAC at 256 kbps, and YouTube audio tracks are typically AAC or Opus. If you need low-bitrate speech or music streams, HE-AAC keeps things listenable where MP3 would fall apart. AAC is also the standard companion to H.264 video in MP4 files, which is why extracting audio from a video almost always yields an AAC track.
Compatibility
AAC support is excellent on modern hardware: iPhone, iPad, Android, Windows, macOS, game consoles, and browsers all decode it natively. Where it lags MP3 is the long tail- Older car stereos, cheap MP3 players, and some embedded devices only handle MP3. When a file must play absolutely anywhere, MP3 still wins.
Converting AAC on mp3.now
Use the AAC to MP3 converter (or M4A to MP3 if your file has the .m4a extension) to maximise device compatibility. To go the other way, MP3 to AAC repackages an MP3 for Apple-centric workflows. Since both formats are lossy, convert from the original source when you can, and pick a generous bitrate- 320 kbps for music- To minimise added loss. If you are not sure what a file contains- .aac, .m4a, and .mp4 extensions all turn up in the wild- Just upload it; the converter identifies the stream and handles it automatically.