From Hydrogenaudio Knowledgebase
APEv2 is a tagging format derived from APEv1 originally developed for MPC audio files, and is now also used in Monkey's Audio, WavPack and OptimFROG. It can also be used with other formats when using programs like foobar2000 or Tag (a program that can create and read tags). It is comparable in functionality to Vorbis comment. Just like Vorbis comment it defines some standard fields, but it also defines some formats for those fields (like what a date should look like).
APEv2 also differs from Vorbis comment in the way lists of values are handled. Suppose a certain song has two artists. In Vorbis comment this will result in two ARTIST entries, but in APEv2 this will result in one Artist field with the two artists separated by a null character (a byte with the value zero).