Eh, it's pretty easy.
4 is read
2 is write
1 is execute
a permission has 4 "bits" associated with it. a common permission is 0755. starting at the right and reading left (which is how permissions are actually read - more on this in a minute), it's world-executable and world-readable (5), group executable and group readable, and owner readable, executable and writeable. anything not specified is interpreted as a leading zero. for example, if you do a
chmod 55 filename
the permissions will be 0055.
chmod 755 filename
and the permissions will be 0755. the leading 0 is used for setting stickybits and the like. you don't need to worry about it unless you're doing suid stuff.
anyway, once you spend some time with it, it'll become pretty simple. 4 read, 2 write, 1 execute. add 'em up.
