The first important consideration is, who owns a file. In Linux, the owner will be a user that has a local user account on the system, while files will also be assigned to a group, making it a sort of double ownership, that allows for rather sophisticated permission setting.
To find the owner of a file, type the following into a terminal:
stat -c "%U %G" /path/to/file
which will return a result like
For example, if I have a file /home/beowulf/somefile.txt, owned by the user beowulf, and the localusers group, the command, and its result, would look like this:
stat -c "%U %G" /home/beowulf/somefile.txt
There are numerous other ways to figure out a file's ownership information, the ls command being one of the most commonly used approaches. Even the stat command would have a lot more output if not the -c "%U %G" bit, which ensured that only the relevant information will be displayed. This is useful for demonstrative purposes, like here, or when you just don't care to find the only pieces of data that really interest you, buried in a lot of text.
Each file and directory (which are treated as files), can have three types of permissions
read or r means the file's contents can be read and listed
write or w means the file's contents can be written and modified
execute or x means the file is executable, such as a binary programme or a script
If any of the above permissions is denied on a file, it will usually be marked by dash character (-) in place of the permission's letter.
There is also a special permissions flag on each file. Mostly you will see it set as - meaning no special permissions, or d indicating that the file is, in fact, a directory, or l, meaning that the file or directory is, in fact, a symbolic link. Being a directory is not technically a permission, but a useful piece of information for both you and the system, as in Linux (and other UNIX-like systems) directories are treated as files.
Other special permissions can appear in the place of the executable bit:
s - Indicates setuid/setgid of user or group respectively. also means x is set.