Ghost Shadow did a good job covering it.

#1: Yes
#2: All software must be software made for mac. You can't use software made for mac on a pc, and you can't use software made for pc on a mac. However, many programs have both Mac and PC versions, and for those that don't, there is almost always an alternative. There are other file sharing apps though. Limewire works on a mac.
#3: Yes
#4: Yes...Usually if the file is in the native format of the program it is created in...say a Word Document, it can be opened in both versions of it (mac and pc). You can also "print to" a PDF on macs, which are files that can be opened using the free Adobe Acrobat Reader. You could then print from that.