newbie mp3 file name question

When an mp3 file is listed in LimeWire's search window, its name is displayed
like "xyz", but after I download it (on a Macintosh) its file name ends with
# followed by a five-character code, like "xyz#123A5.mp3". Where is that code coming from and what does it mean?

I get the same thing, and then no program will open it. It is very distressing.