A backlink is an incoming link to your website, for example, situated on another webspace. The number of backlinks indicates the popularity or importance of your website/blog. The number of backlinks is not decisive for the pagerank rating, and 1000 bad incoming links doesn’t make a website more popular neither rise up its PR, it’s totally the opposite. The quality of a single backlink is determinant instead.
What’s bad and what’s good?
Google is introducing warnings in its search results, notifying users about websites which are known to distribute "badware", malicious software such as spyware, deceptive adware, viruses and trojans that disrupt the smooth running of computers. Google search users will be warned before they enter suspect websites: "The site you are about to visit may harm your computer!"
Are bad all the backlinks against google policy (in the case of google). A backlink is good when it is on a popular or high ranked website or a stable webspace; are especially good the incoming links from domains .edu and .gov
Now that we have a global view of the backlink term, even if not so detailed, we can introduce some useful tools: Backlink Watch: http://www.backlinkwatch.com
The script above list the main backlinks of a website. But the most useful things is the nofollow attribute displayed under the column Flag.
Link Exchange case: Someone asks to you to make a link exchange. He put your link on his webspace and you put his on your webspace, simple, not?
But, in the case that this someone have a nofollow attribute for the links, your webspace will not get benefit from the exchange.