Apparently, something changed in the ls command with the release of Leopard. I don't remember seeing this kind of report on Tiger, although it looks like the features already existed since long time. Now, take this information with a grain of salt, as I am not a MacOSX expert, just a (very busy) occasional tinkerer on this OS.

drwxr-xr-x   5 stefano  stefano     170  9 Ott 10:11 Programs
drwxr-xr-x+  4 stefano  stefano     136  1 Ago 01:48 Public
-rw-r--r--@  1 stefano  stefano  221814 20 Ott 20:12 executable network.eps

This is an extract of an ls run in my home directory. As you can see, the long -l option produces an output containing two additional characters in the file mode field: an "at symbol" and a "plus symbol" permission.

According to the manual page, the plus symbol means that the file has extended security information, meaning ACLs. It is possible to print out this information with the option -e to the ls command.

stefano:~ stefano$ ls -led Public/
drwxr-xr-x+ 4 stefano  stefano  136  1 Ago 01:48 Public/
 0: group:everyone deny delete

Of course, you can also change the ACL information using chmod

stefano:~ stefano$ chmod +ai "guest allow write" Public/
stefano:~ stefano$ ls -le Public/
total 0
drwxr-xr-x+ 4 stefano  stefano  136  1 Ago 01:48 Public/
 0: group:everyone deny delete
 1: user:Guest inherited allow add_file

And you can remove the ACLs

stefano:~ stefano$ chmod -a# 1 Public
stefano:~ stefano$ chmod -a# 0 Public
stefano:~ stefano$ ls -led Public/
drwxr-xr-x  4 stefano  stefano  136  1 Ago 01:48 Public/

As you can see, the plus symbol disappears. Ideally, you can do these changes also from the Finder, Information window. However trying to do this lead to a crash of the Finder on my machine. Don't know if this behavior is due to my setup or an actual bug.

Then, what about the at symbol? This feature is undocumented, but if you add the -@ option to ls -l you will obtain the meaning:

stefano:~ stefano$ ls -l@ executable\ network.eps
-rw-r--r--@ 1 stefano  stefano  221814 20 Ott 20:12 executable network.eps
    com.apple.FinderInfo    32
    com.apple.ResourceFork 547102

So, apparently there are extended attributes, and this is the meaning of the at symbol. the first entry is the name of the extended attribute, the second entry is the size of the contents of the attribute.

You can peek into the contents of the attributes with the command xattr:

stefano:~ stefano$ xattr -p com.apple.ResourceFork executable\ network.eps >unknown_content
stefano:~ stefano$ file unknown_content
unknown_content: MS Windows icon resource

Apparently, the resource fork contains a icon file. ResKnife also confirms it. If I understood correctly, the old file/..namedfork/rsrc resource fork is mapped to com.apple.ResourceFork attribute, but you are welcome to correct me in the comments. I think I should read this book for a better understanding.

Here some relevant links: