Most audio files on the network being shared are mp3's. wma & m4a/mp4 & Ogg would be next popular at a guess. Then would be real files. You will not find high numbers of lossless file format files being shared. AIFF or WAV or FLAC, APE, WV, etc. are all lossless formats but not too commonly shared.

I personally share about a dozen albums of songs in AIFF format. One or two albums as apple lossless (m4a).
Lossless means no information was thrown away during the conversion into that format. http://en.wikipedia.org/wiki/Audio_file_format