All mpeg formats are "lossy" formats. DivX (patented), uses lossy MPEG-4 Part 2 compression, XviD (open source & uses elements of H.263), 3ivx are examples of some makers of MPeg4 formats.

H.264 (ITU-T H.264 standard) is mpeg4 part 10 & from what I've read is one of the very best mpeg4 formats now being commonly used. According to that link HD-DVD format of the DVD Forum & The Blu-ray Disc are considering using H.264 instead of the present mpeg2 format which is used for DVD's. ie: the higher the mpeg version, the more efficient it is at converting with size & quality the factors. H.264 is better than DivX.

Yes mac has lossless formats for video. QuickTime has a no. of options for lossless. It's own & the animation format being ones I'm aware of.

I'm not sure why someone would want to convert an mp3 to mp3pro. Mp3Pro's abilities are aimed at files between 50-100 kbps, so anything larger is not as efficient as other converters. Even iTunes mp3 codec surpasses it at bitrates higher than about 140. And Lame of course is the best of them all at anything larger than 128 kbps (I'll leave m4a out of this discussion.) For iTunes, there are 3rd party plug-ins for both Lame & Ogg Vorbis. In the very earlier days of mp3, there were no converters around for anything above 128 kbps but times have changed. And of course, mp2 was originally used in file sharing but that's totally gone by the wayside now.

Your reference to "Newbies" which generally means windows users who have what comes with their systems. Which is wmp & its formats; What you should know about WMA/WMV Files Not the greatest of audio or video formats to say the least. Forgive them ! for they know not what they do. lol These people are also often attracted by realplayer & its very highly patented formats. Don't we just love trying to convert or use rp files. I don't believe it's safe to downld zipped or rar files at present. Since the equivalent of Overpeer loves to pollute the network with such files. Oh & newbies also generally use the default settings for conversion which are generally around the 128 kbps for audio, etc. Audiophile people look with disgust. lol
Time to stop my rambling & back to the point. A better definition of multimedia files, particularly video files would be beneficial for "most" people. So they know exactly what it is they have found in their search results. Not a vague description like avi or mpeg or mov.