From the internet:

There is a standard format for Sokoban level files. It is simply a 
text file that uses the characters listed below to describe each 
level. You can download level files from other Sokoban sites, or 
create your own, as long as they are in this format.

<space> - empty space 
      $ - box
      . - target space
      * - box on target space
      @ - player starting position
      + - player starting on target space
      # - wall

Example level:

######
#@   #####
# $ $  ..#
##########


You can put more than one level in a single text file by separating 
each level by a blank line. Any line which contain characters other 
than those listed above, will be ignored. This allows you to include
 comments and descriptions along with each level in the file.