File Permissions in Ubuntu

Firstly you should know that:

1. + means add this permission to the other permissions that the file already has.
2. = means ignore all permissions, set them exactly as I provide.
   So all of the "read, write, execute, sticky bit, suid and guid" will be ignored and only the ones provided will be set.
3. read = 4, write = 2, execute = 1
   Here is the binary logic behind it (if you're interested):

Using +x you are telling to add (+) the executable bit (x) to the owner, group and others.

• it's equal to ugo+x or u+x,g+x,o+x
• When you don't specify which one of the owner, group or others is your target, in case of x it will considers all of them. And as @Rinzwind pointed out, it's based on umask value, it adds the bit to the ones umask allows. remember if you specify the target like o+r then umask doesn't have any effect anymore.
• It doesn't touch the other mods (permissions).
• You could also use u+x to only add executable bit to the owner.

Using 755 you are specifying:

• 7 --> u=rwx (4+2+1 for owner)
• 5 --> g=rx (4+1 for group)
• 5 --> o=rx (4+1 for others)

So chmod 755 is like: chmod u=rwx,g=rx,o=rx or chmod u=rwx,go=rx.