In a three digit permission, the 1st digit indicates permissions granted the file's owner, the 2nd digit indicates permissions granted to the users in the file's group, and the 3rd digit indicates permissions granted to everyone else. When a digit is 7, that class of users has read, write and execute (for regular files) or search (for directories) permission. When a digit is 5 that class of users does not have write permission.
The difference between 777 and 775 is that users who are not the file's owner and who are not in the group of that file do not have permission to write to the file if the mode is 775, but do have permission to write to that file if the mode is 777. (If the file is a directory, write permission is needed to create a file in that directory and to remove a file from that directory.)
The Following User Says Thank You to Don Cragun For This Useful Post: