avoid links to web spammers or "bad neighborhoods" on the web, as your own ranking may be affected adversely by those links
A bad neighborhood on the Web is any website that uses any dirty tricks to try and increase their site's rankings or do harm to customers reading the site. These include:
All of these things will get a site penalized or banned from Google and other search engines, and if you link to these sites, your site will be penalized.
How to Check if a Site is a Bad Neighborhood
Search Google for links to that site. If there are none, chances are the site has been banned. In the Google search engine, type:
If no results are found, it's not a good idea to link to that site, as it is probably considered a bad neighborhood.