How do I use chmod? / What is chmod?
It has some nice information on what chmod actually is.
Just some basic info for people from the site:
chmod (change mode)
permissions for the owner, the group and the world, respectively
digits include 4 (read only), 5 (read and execute), 6 (read and write) and 7 (read, write and execute).
so 777 means that owner, group and world have read write and execute permissions.
hope this lifts some of the fog of confusion