What is + in Linux file/direcory permission column?

Recently when I was surfing around the /var/log directory I've noticed some directory has a '+' symbol in the file permission column while other have a '.' (dot) in their file permission column. Does anyone know what the '.' (dot) or '+' means in the file permission?

enter image description here

Solution

Quoted from man acl:

• For files that have a default ACL or an access ACL that contains more than the three required ACL entries, the ls(1) utility in the long form produced by ls -l displays a plus sign (+) after the permission string.

i.e. it means Access Control Lists in effect.