What is M4A?
AAC audio in an MP4 container, which is what iTunes ripped to and what most podcast apps hand you. It sounds better than MP3 at the same bitrate. It's also the file that gets refused by car stereos and upload forms often enough to keep this converter busy.
What is AIFF?
Apple's uncompressed format, the Mac counterpart to WAV, and still what Logic and older Pro Tools sessions expect. Its one genuine oddity is storing the sample rate as an 80-bit extended float, a data type almost nothing else in computing still uses.