Google advises you to mark specific types of links with the nofollow-attribute. These restrictions can be found within the Google webmaster guidelines.
Search engine guidelines require machine-readable disclosure of paid links in the same way that consumers online and offline appreciate disclosure of paid relationships (for example, a full-page newspaper ad may be headed by the word “Advertisement”).– Google Content Guidelines
– Google Content Guidelines
If one of the following types of links is placed on your own website or linked to another source, it must be marked with the nofollow value:
- Affiliate Link
- Paid Link
- Paid Advertising (as both text or images)
- Link in/from press releases
- Link in/from Advertorials
- Link in an article which has only a minimal topical connection to the target page
- untrustworthy content
- Link in a widget
- Links to pages which only consist of a form (for example “login” or “register here”)
