I came across this excellent tutorial on Linux file permissions:
http://ryanstutorials.net/linuxtutorial/permissions.php

Linux file permissions can be quite cumbersome and confusing at times, but once you get to grips with them they are extremely useful. I particularly liked the table mapping octal to binary in the above tutorial. I have rewritten the table below mapping the octal number to the corresponding read, write execute permissions for ease of use.

Octal
Read
Write
Execute
0
-
-
-
1
-
-
x
2
-
w
-
3
-
w
x
4
r
-
-
5
r
-
x
6
r
w
-
7
r
w
x