AAC+ (Advanced Audio Coding Plus) is a compressed audio format defined in MPEG-4 Part 3 (ISO/IEC 14496-3). It is a successor to AAC, and is designed to be more efficient than AAC for low bitrates. It is also known as HE-AAC (High Efficiency AAC).
An AAC+ file may contain only the raw AAC+ format, or it may use a multimedia container format such MP4 or QuickTime. AAC+ is often used for the audio component of a video file.
Differs from audio/aac and has lower browser compatibility for playback. The Wikipedia link has the most information I could find.