files with ".app" are a collection of files, that work as an executable. I can go into much detail about that, but to keep it plain and simple ".app" is the Mac equivalent to .Exe files.
I can't really say which version of OSX would run well on a G3. My first Mac is already a G5, so I don't know much about the speed of older models.
I can't really say which version of OSX would run well on a G3. My first Mac is already a G5, so I don't know much about the speed of older models.