I have read some article said that buiding link with reciprocal link directory not only profitable but also harmful ? I have already build link for one of my websites with directories, and PR of it is 5. Other sites not use that SEO , PR is zero . Why that ? And i am confusing about back links and popularity links, explain clear for me
Backlinks and popularity links are the same thing. As for submitting to reciprocal directories, there's no problem there but go for ones that are in your niche and check that they only list quality sites.
I think if you're going to do a recip link directory you have to check the reputation. How long have they been around, what do they have for ads, any annoying crap, do they advertise in unfavorable places or spam? PR isn't everything for sure, try only to list in very reputable directories for the best results.