You must keep in mind Linux is designed to be a multi-user environment. In an environment with more than one users(like Linux), it is crucial to have a secure system for deciding which files are yours and who can fiddle with them. So, file permissions are defined separately for users, groups, and others as following:
User: The username of the person who owns the file. By default, the user who creates the file will become its owner.
Group: The usergroup that owns the file. All users who belong into the group that owns the file will have the same access permissions to the file. This is useful if, for example, you have a project that requires a bunch of different users to be able to access certain files, while others can't. In that case, you'll add all the users into the same group, make sure the required files are owned by that group, and set the file's group permissions accordingly.
Other: A user who isn't the owner of the file and doesn't belong in the same group the file does. In other words, if you set a permission for the "other" category, it will affect everyone else by default. For this reason, people often talk about setting the "world" permission bit when they mean setting the permissions for "other."
That was the categories where you set permissions. The following are THE permissions and they are defined separately for the file's owner, group and all other users.
Read permission. On a regular file, the read permission bit means the file can be opened and read. On a directory, the read permission means you can list the contents of the directory.
Write permission. On a regular file, this means you can modify the file, and write new data to the file. In the case of a directory, the write permission means you can add, remove, and rename files in the directory. This means that if a file has the write permission bit, you are allowed to modify the file's contents, but you're allowed to rename or delete the file only if the permissions of the file's directory allow you to do so.
Execute permission. In the case of a regular file, this means you can execute the file as a program or a shell script. On a directory, the execute permission (also called the "search bit") allows you to access files in the directory and enter it, with the cd command, for example. However, note that although the execute bit lets you enter the directory, you're not allowed to list its contents, unless you also have the read permissions to that directory.
You can view the access permissions of a file by doing the long directory listing with the ls -l command.
$ ls -l
The first character can be any of these(type of file):
d = directory
- = regular file
l = symbolic link
s = Unix domain socket
p = named pipe
c = character device file
b = block device file
For each file you can add the following:
r = read permission
w = write permission
x = execute permission
- = no permission
