Executable files in windows and linux???

I know that a system with windows (without strong anti-virus) would be
easily pulled down by the viruses in the LAN whereas in linux it's not so..
I heard that in Windows the files are identified by their file extension
like .exe,.com,etc(and hence the viruses with .exe extensions run easily on
windows system).. whereas in Linux it is by the file type and hence
whatever executable file is there  it won't run without the permission of
the superuser.
what about the executable files in windows and in linux??
In windows any file with .exe extension is an executable file..
but in Linux the extension doesn't matters...then how is the executable file
known/identified??
Please explain.....