Server Message Block (SMB) or Samba is the file-sharing utility of Linux and older Windows systems. The clubs are the big wooden kind. It works by exposing folders to the network for authenticated (or not, as the case may be) users. There are a number of good practices here that are frequently ignored, which makes it a prime target for testers.
Enabled guest accounts
Exposing sensitive folders
Running out-of-date versions of Samba
Allowing writeable directories