In order to remove a file, you must have write permission to the directory where the file is located. When removing a file that you do not have write permission on, a prompt will come up asking you whether or not you wish to override the write protection.
The -r option is very handy and very dangerous. -r can be used to remove a directory and all its contents. If you use the -i option, you can possibly catch some disastrous mistakes because it'll ask you to confirm whether you really want to remove a file before going ahead and doing it.
