A paid link is simply a link that you pay for. Search engines consider links as votes for the popularity and relevance of your site, so paid links that aren't explicitly declared as not passing link juice (using the nofollow attribute, etc) are treated as spammy by the search engines (if they can detect them). Generally spammy links won't benefit your site's ranking, but they generally won't hurt it either.

With paid links on web directories, the value of the links really depends on how much editorial control is exercised by the directory owner. If they approve all links without checking and rejecting the bad ones, then the search engines will consider that directory a link farm, and will consider all the links on it as spammy. If links are only approved if they meet a high standard of quality and relevance, then the search engines will likely regard the directory as an authority site, and will give the links on it some link juice.

Instead of paying for links you should try build some for yourself. Finding places for quality links isn't so difficult these days...

They are seen as spam because they are - if used without nofollow attribute - regarded as a means of unnaturally trying to usurp SERPs. Any sites accepting paid links therefore run the risk of a Google penalty which is not something you want to experience.

Its better to follow Gs rule than taking any risks.

Well, it can pay to circumnavigate them... in the short term, at least. But Google has a habit of catching up with those who don't play ball, and will punish them, sometimes for a very long time.

They seem spam as they are not natural and they are blackhat SEO and Google can find out it easily. Try to concentrate on natural ways to improve PR.

I'm not sure Google always finds out easily. I know of many sites that have use spam tactics for a long time. Some I've even reported and yet they still benefit from their underhand antics. Google does have a tendancy to catch up, but not always quickly.

yes I agree, But why taking the risk if there is a right ways to take.

You could say the same about the stock market. People take the risk because they believe the rewards are potentially greater.