Google has updated their interpretation of a link scheme here. In summary here is what they define it as:
- Links intended to manipulate PageRank
- Links to web spammers or bad neighborhoods on the web
- Excessive reciprocal links or excessive link exchanging (”Link to me and I’ll link to you.”)
- Buying or selling links
They are really starting to crack down and reciprocal linking and buying links. That have added the Excessive part, which could mean that you would be ok with at least a few reciprocal links, just don’t go overboard with it. One thing to always keep in mind when adding links to your page, is if the link is valuable to your users. If it is not a valuable link, then Google can also spot those links and devalue them, or view your site as a link scheming website.