Here we go:
MP3: MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression.
It is a common audio format for consumer audio storage, as well as a de facto standard of digital audio compression for the transfer and playback of music on digital audio players.
The use in MP3 of a lossy compression algorithm is designed to greatly reduce the amount of data required to represent the audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners. An MP3 file that is created using the setting of 128 kbit/s will result in a file that is about 1/11 the size of the CD file created from the original audio source. An MP3 file can also be constructed at higher or lower bit rates, with higher or lower resulting quality.
-->The compression works by reducing accuracy of certain parts of sound that are considered to be beyond the auditory resolution ability of most people. This method is commonly referred to as perceptual coding.
It uses psychoacoustic models to discard or reduce precision of components less audible to human hearing, and then records the remaining information in an efficient manner.
AAC: Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates.
AAC supports inclusion of 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 16 low frequency effects (LFE, limited to 120 Hz) channels, up to 16 "coupling" or dialog channels, and up to 16 data streams. The quality for stereo is satisfactory to modest requirements at 96 kbit/s in joint stereo mode; however, hi-fi transparency demands data rates of at least 128 kbit/s (VBR).
AAC is also the default or standard audio format for iPhone, iPod, iPad, Nintendo DSi, iTunes, DivX Plus Web Player and PlayStation 3. It is supported on PlayStation Portable, Wii (with the Photo Channel 1.1 update installed), Sony Walkman MP3 series and later, Sony Ericsson; Nokia, Android, BlackBerry, and webOS-based mobile phones. AAC has also seen some adoption on in-dash car audio especially on high-end units such as the Pioneer AVIC series.
Now that's why i keep most of my tracks on .wma or .aac with 128~160kbps for highest quality, as you read the mp3 "works by reducing accuracy of certain parts of sound that are considered to be beyond the auditory resolution" and also reducing the size of tracks but since storage size isn't a matter to consider nowadays... (it was in the past).
So, the results from an original .wma 96kpbs converted to .mp3 128kbps will be 'good' and but will sound better when converted to .aac 128~160kbps.