The important thing to remember is that file permissions are read/write/execute for owner,group,everyone.
So if a file by the name of mymidgetporn.avi has permissions of rwxr-x---
and its ownership is Myusername Users
This means that the owner is Myusername and the group ownership is Users
Let's break the permissions down now. The above can be broken into 3 sections:
That would mean that the user Myusername can read, write, an execute the file.
The second set is "r-x" indicates that the group "Users" can read and execute the file, but cannot write to it, overwrite it, or delete it.
And finally, nobody outside the group can read, execute, or write to this file.
Now....to set file permissions, do some reading on the chmod command.
Hope this helps.