Which one is better? Directory back links or static links? I have read many time about both links. and my second Question is that why blog links are not good for site? is it right? many guys fear for link exchange. I have tried and focus on this matter. why so?
Im not sure what a static link is to be honest. Ive not heard that about blogs either. Personally i think what does no favours for anything from blogs, to directories to social networks is that everything gets exploited, black hat types find something and debowel it. Theres always the taking it too far method that leads to the ruining of anything decent so all the decent webmasters working with something get to suffer for what someone else does. Examples would be the obvious with diectories. Blog networks to sell links across thousands of blogs. Sadly the next best marketing methods will get exploited too.
Directory links and blog links .. both are fine if you keep your eyes open while getting a backlink on them. If you are getting a backlink from a link farm or a spam blog it's not going to help. But otherwise both are useful.
Both are equally useful...but a directory backlink is easier to obtain than a link from a static site if your site has the right type of content which a directory can approve. And besides, why would a competitor of your site want to link to your site? A directory is neutral in that aspect, it can list both yours and your competitor's site without any issues.