Advanced Audio Compression (AAC), of which are different types:
LC (low complexity), HE (high efficiency) to name a few.
To encode your audio to be compatible with most audio players, use the LC compression type.
High Efficiency (HE AAC) uses SBR technology (not too different as mp3PRO) .
if the player knows about HE the extra frequency information can be decoded, otherwise half the audio quality will be missing on playback.
A newer standard again for ultra low bitrates is AAC+ or AAC HE v2, this uses parametric stereo for even lower bitrates.